[Java][google-api-client] Empty POST requests don't send content type; List type query params

[charlescapps]

Description

In the client support I added for google-api-client, there are a few minor bugs needing fixing.

• Empty POST requests can fail because it doesn't send the Content-Type
• Query params aren't used correctly if they are List<String> type
• Support InputStream type for the request body; highly useful for proxy services

Swagger-codegen version

2.3.1 (I hope to eventually port these templates to Swagger 3.0.0).

Swagger declaration file content or url

n/a - this really depends on how the server is configured to require the Content-Type header, but here is an example snippet of the endpoint that was failing and takes an empty POST request:

"/forms/{id}/publish" : {
      "post" : {
        "tags" : [ "Forms" ],
        "summary" : "Publish a Form",
        "description" : "",
        "operationId" : "publishForm",
        "consumes" : [ "application/json" ],
        "produces" : [ "application/json" ],
        "parameters" : [ {
          "name" : "id",
          "in" : "path",
          "required" : true,
          "type" : "integer",
          "format" : "int64"
        } ],
        "responses" : {
          "200" : {
            "description" : "successful operation",
            "schema" : {
              "$ref" : "#/definitions/Form"
            }
          }
        }
      }
    }

Command line used for generation

config.json

{
  "library": "google-api-client",
  "artifactId": "${CLIENT_LIB_ARTIFACT_ID}",
  "groupId": "${SERVICE_GROUP_ID}",
  "artifactVersion": "${SERVICE_VERSION}"
}

# Generate the code!
java -jar swagger-codegen-cli-2.3.1.jar \
  generate -i swagger.json \
  -l java \
  -o ${SWAGGER_BUILD_DIR} \
  -c config.json \
  --api-package "${CLIENT_LIB_PACKAGE}.api" \
  --model-package "${CLIENT_LIB_PACKAGE}.model" \
  --invoker-package ${CLIENT_LIB_PACKAGE}

Steps to reproduce

Use the swagger client to contact an endpoint that takes no request body (empty POST) but the JAX-RS annotations are configured with @Consumes({"application/json"}) so that a content-type must be set.

Result: get a 415 : Unsupported media type error.

Related issues/PRs

#6825
#6838

Suggest a fix/enhancement

I'm fixing it! About to put up a pull request.