file.serialize JSON w/merge_if_exists fails if top-level is not a dict

Description of Issue/Question

file.serialize has the merge_if_exists option.
When serializing to JSON, it apparently assumes the top level to be a dict.
While this is of course extremely common, it's not always true, and when it's not, the serializer crashes.

Steps to Reproduce Issue

SLS:

my-json-list:
  file.serialize:
    - name: /tmp/list.json
    - formatter: json
    - merge_if_exists: True
    - dataset:
        - tea
        - pot

# cat /tmp/list.json
[  "tea", "kettle" ]

          ID: my-json-list
    Function: file.serialize
        Name: /tmp/list.json
      Result: False
     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1746, in call
                  **cdata['kwargs'])
                File "/usr/lib/python2.7/dist-packages/salt/loader.py", line 1704, in wrapper
                  return f(*args, **kwargs)
                File "/usr/lib/python2.7/dist-packages/salt/states/file.py", line 5144, in serialize
                  merged_data = salt.utils.dictupdate.merge_recurse(existing_data, dataset)
                File "/usr/lib/python2.7/dist-packages/salt/utils/dictupdate.py", line 82, in merge_recurse
                  return update(copied, obj_b, merge_lists=merge_lists)
                File "/usr/lib/python2.7/dist-packages/salt/utils/dictupdate.py", line 35, in update
                  raise TypeError('Cannot update using non-dict types in dictupdate.update()')
              TypeError: Cannot update using non-dict types in dictupdate.update()
     Started: 11:32:11.537613
    Duration: 3.339 ms
     Changes:   

Versions Report

Salt Version:
           Salt: 2016.11.4
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: 2.4.2
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.8
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: 1.0.3
   msgpack-pure: Not Installed
 msgpack-python: 0.4.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.12 (default, Nov 19 2016, 06:48:10)
   python-gnupg: Not Installed
         PyYAML: 3.11
          PyZMQ: 15.2.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4
 
System Versions:
           dist: Ubuntu 16.04 xenial
        machine: x86_64
        release: 4.10.0-20-generic
         system: Linux
        version: Ubuntu 16.04 xenial

Start with salt/states/file.py at serialize and trace the merge_if_exists path into salt/utils/dictupdate.py, especially merge_recurse and update. Reproduce the issue with the provided SLS and confirm the expected behavior for a JSON list when an existing file is present; done means the non-dict case no longer crashes and relevant tests pass.