C# Custom JSON Content-Types not working

Description

When generating an SDK for API endpoints with custom json content-types, the current logic in the ApiClient class overrides the content-type to 'application/json' if the type of the request body is string.

Swagger-codegen version

Latest stable version 2.2.3

Command line used for generation

-jar sdk-generate.jar generate -l csharp -i -o output -DmodelTests=false
-DapiTests=false",

Steps to reproduce

1. Include custom consumes / produces in the path definition for a swagger document.

eg: "application/vnd.ed-fi.student.student-enrollment-readable-includes-embedded-object.readable+json"

2. Generate sdk and compile.

3. Make a request to the api using the sdk and capture using fiddler.

4. Inspect body and notice the content type is not the custom json type but rather 'application/json'

Related issues/PRs

None found

Suggest a fix/enhancement

ApiClient.mustache in the csharp resource has been updated locally to reflect the java implementation of Content-Type selection (Was going to open a prq but wanted to mention it here first). Regardless of body type (string or byte[]), I think we should use the content-type the SDK was generated with and not default to Application/Json in certain scenarios. The logic found in the java mustache file is as follows and was replicated for csharp.

1. If none we're provided, default to application/json.
2. If multiple we're provided, iterate over the list and if any 'IsJsonMime', return it.
3. If not just return the first Content-Type in the list.

Start with the C# ApiClient.mustache template and compare its Content-Type selection with the Java mustache implementation described in the issue. Generate a C# SDK from a Swagger document using the custom JSON content type, compile it, and capture a request to verify that the generated client preserves the custom type instead of using application/json.