Actions with immutable=true, default set and enum cannot be run from UI

SUMMARY

While trying to run example from https://github.com/StackStorm/st2/blob/ddd3aba57d8cccaaaef64babd07dd48e0b68c42a/contrib/examples/actions/weather.yaml from the web UI I ran into an issue when the call to API would fail with

{
    "faultstring": "Override of immutable parameter(s) [u'method'] is unsupported."
}

Running the same action from CLI worked just fine. Also, removing enum helps.

ISSUE TYPE

• Bug Report

STACKSTORM VERSION

Paste the output of st2 --version:

st2 2.9.0, on Python 2.7.10

OS / ENVIRONMENT / INSTALL METHOD

RPM packages on CentOS 6.

STEPS TO REPRODUCE

---
description: Look up weather via http://openweathermap.org
# Free weather API is provided under the terms of the Creative Commons Attribution-ShareAlike 4.0 Generic License.
# Terms of service http://openweathermap.org/terms
enabled: true
entry_point: ''
name: weather
parameters:
  auth:
    type: string
  headers:
    type: object
  method:
    default: GET
    type: string
    enum:
    - GET
    - POST
    - PUT
    - DELETE
    immutable: true
  params:
    type: string
  timeout:
    default: 60
    type: integer
  url:
    default: http://api.openweathermap.org/data/2.5/weather?q={{zip}}&units=imperial
    immutable: true
    type: string
  zip:
    type: string
runner_type: "http-request"

Going to st2 web and trying to execute this action fails. The UI is adding method: GET into params passed to st2api, which it rejects with the explanation shown above and below.

EXPECTED RESULTS

Expect it to be run successfully both from the UI and CLI.

ACTUAL RESULTS

Got 400 Bad Request with

{
    "faultstring": "Override of immutable parameter(s) [u'method'] is unsupported."
}

Start with contrib/examples/actions/weather.yaml and reproduce the request from st2 web, then compare it with the CLI invocation and the st2api response. Trace where the UI adds the default enum value for immutable method; done means the weather action runs successfully from both UI and CLI without the immutable-override 400.