Prepend with header not working in test=True

Description of Issue/Question

Setup

Ubuntu 18.04, Salt 2018.3.3

Steps to Reproduce Issue

• Write a state prepending some text to a file with the header parameter set to true.
• Apply the state once
• Try to reapply in test=True => it shows changes
• remove test=True => it doesn't change anything

Example state :

Create file:
  file.exists:
    - name: /etc/testing-prepend

Add nice header:
  file.prepend:
    - name: /etc/testing-prepend
    - header: True
    - text: |
          ################################
          ##### THIS IS A NICE HEADER ####
          ################################
    - require:
      - file: Create file

Example output :

# salt-call state.apply test-prepend-header --state-output=changes
local:
----------
          ID: Create file
    Function: file.exist
        Name: /etc/testing-prepend
      Result: True
     Comment: Created empty file /etc/testing-prepend
     Started: 22:05:48.401595
    Duration: 5.676 ms
     Changes:   
              ----------
              new:
                  /etc/testing-prepend
----------
          ID: Add nice header
    Function: file.prepend
        Name: /etc/testing-prepend
      Result: True
     Comment: Prepended 3 lines
     Started: 22:05:48.407609
    Duration: 1.909 ms
     Changes:   
              ----------
              diff:
                  --- 
                  +++ 
                  @@ -0,0 +1,3 @@
                  +################################
                  +##### THIS IS A NICE HEADER ####
                  +################################

Summary for local
------------
Succeeded: 2 (changed=2)
Failed:    0
------------
Total states run:     2
Total run time:   7.585 ms
# cat /etc/testing-prepend 
################################
##### THIS IS A NICE HEADER ####
################################
# salt-call state.apply test-prepend-header --state-output=changes test=True
local:
  Name: /etc/testing-prepend - Function: file.exists - Result: Clean Started: - 22:12:03.129254 Duration: 0.43 ms
----------
          ID: Add nice header
    Function: file.prepend
        Name: /etc/testing-prepend
      Result: None
     Comment: File /etc/testing-prepend is set to be updated
     Started: 22:12:03.130029
    Duration: 1.858 ms
     Changes:   
              ----------
              diff:
                  --- 
                  +++ 
                  @@ -1,3 +1,6 @@
                   ################################
                   ##### THIS IS A NICE HEADER ####
                   ################################
                  +################################
                  +##### THIS IS A NICE HEADER ####
                  +################################

Summary for local
------------
Succeeded: 2 (unchanged=1, changed=1)
Failed:    0
------------
Total states run:     2
Total run time:   2.288 ms
# salt-call state.apply test-prepend-header --state-output=changes
local:
  Name: /etc/testing-prepend - Function: file.exists - Result: Clean Started: - 22:10:44.662027 Duration: 0.447 ms
  Name: /etc/testing-prepend - Function: file.prepend - Result: Clean Started: - 22:10:44.662811 Duration: 0.745 ms

Summary for local
------------
Succeeded: 2
Failed:    0
------------
Total states run:     2
Total run time:   1.192 ms
# cat /etc/testing-prepend 
################################
##### THIS IS A NICE HEADER ####
################################

Versions Report

# salt-call --versions-report
Salt Version:
           Salt: 2018.3.3
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: 2.6.1
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: 1.0.7
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.15rc1 (default, Nov 12 2018, 14:31:15)
   python-gnupg: 0.4.1
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.2.5
 
System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-34-generic
         system: Linux
        version: Ubuntu 18.04 bionic

We use test=True a lot, and it looks like an actual bug. We also don't have a documentation for the header parameter.

Any ideas ?

Start at the file.prepend state entry point and reproduce the supplied state on Salt 2018.3.3, first applying it normally and then with test=True. Compare the reported diff with the resulting file contents; done means test mode no longer reports the header as being prepended again, while the issue’s missing header documentation is addressed if included in scope.