[php] 2.3 - fix form params json encoding

[baartosz]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Issue introduced when client was reworked to use guzzle instead of curl in #5190.

Apparently I missed this case earlier. Content of request is not json-encoded if endpoint consumes: application/json and params are in formData. This PR fixes this issue.
I added example of spec causing this issue to FakeApi definition so I could add test for it.

  /fake/jsonFormData:
    get:
      tags:
        - fake
      summary: test json serialization of form data
      description: ''
      operationId: testJsonFormData
      consumes:
        - application/json
      parameters:
        - name: param
          in: formData
          description: field1
          required: true
          type: string
        - name: param2
          in: formData
          description: field2
          required: true
          type: string
      responses:
        '200':
          description: successful operation

[wing328]

@baartosz do you mind adding a new method (e.g. isJsonMime) to determine if a MIME string is "JSON"?

For Java, we've https://github.com/swagger-api/swagger-codegen/blob/master/samples/client/petstore/java/okhttp-gson/src/main/java/io/swagger/client/ApiClient.java#L752-L754

and here are some tests:

swagger-codegen/samples/client/petstore/java/okhttp-gson/src/test/java/io/swagger/client/ApiClientTest.java

Lines 81 to 102 in fc01457

	public void testIsJsonMime() {
	assertFalse(apiClient.isJsonMime(null));
	assertFalse(apiClient.isJsonMime(""));
	assertFalse(apiClient.isJsonMime("text/plain"));
	assertFalse(apiClient.isJsonMime("application/xml"));
	assertFalse(apiClient.isJsonMime("application/jsonp"));
	assertFalse(apiClient.isJsonMime("example/json"));
	assertFalse(apiClient.isJsonMime("example/foo+bar+jsonx"));
	assertFalse(apiClient.isJsonMime("example/foo+bar+xjson"));
	assertTrue(apiClient.isJsonMime("application/json"));
	assertTrue(apiClient.isJsonMime("application/json; charset=UTF8"));
	assertTrue(apiClient.isJsonMime("APPLICATION/JSON"));
	assertTrue(apiClient.isJsonMime("application/problem+json"));
	assertTrue(apiClient.isJsonMime("APPLICATION/PROBLEM+JSON"));
	assertTrue(apiClient.isJsonMime("application/json\t"));
	assertTrue(apiClient.isJsonMime("example/foo+bar+json"));
	assertTrue(apiClient.isJsonMime("example/foo+json;x;y"));
	assertTrue(apiClient.isJsonMime("example/foo+json\t;"));
	assertTrue(apiClient.isJsonMime("Example/fOO+JSON"));
	}

[wing328]

I'll create a separate task to track this.

[wing328]

@baartosz thanks for the PR. I left you a feedback in the code.

cc @arnested

[wing328]

@baartosz when you've time, I wonder if you can contact me via the email address in my Github profile as I've some questions for you (not related to this PR). Thanks.