[Java] String as POST body not supported

Description

Code generator for java language produces a client which does not support using a POST method with a String body and TEXT_PLAIN media type. This can be seen in the serialize method in the generated ApiClient.java:

    /**
     * Serialize the given Java object into request body according to the object's
     * class and the request Content-Type.
     *
     * @param obj The Java object
     * @param contentType The request Content-Type
     * @return The serialized request body
     * @throws ApiException If fail to serialize the given object
     */
    public RequestBody serialize(Object obj, String contentType) throws ApiException {
        if (obj instanceof byte[]) {
            // Binary (byte array) body parameter support.
            return RequestBody.create(MediaType.parse(contentType), (byte[]) obj);
        } else if (obj instanceof File) {
            // File body parameter support.
            return RequestBody.create(MediaType.parse(contentType), (File) obj);
        } else if (isJsonMime(contentType)) {
            String content;
            if (obj != null) {
                content = json.serialize(obj);
            } else {
                content = null;
            }
            return RequestBody.create(MediaType.parse(contentType), content);
        } else {
            throw new ApiException("Content type \"" + contentType + "\" is not supported");
        }
    }

As you can see, if the body (obj) is neither a byte[] nor File, and the content type is not a json type, we go to the exception case.

This file comes from the okhttp-gson version of ApiClient.mustache, which is the default.
I propose that we change that file to have the same serialize method as the generic Java one (which is also the same as e.g. the resteasy version). I suspect the code above is just an old leftover that was already fixed for the other Java library versions.

Would a PR to that effect be accepted?

Swagger-codegen version

This affects 2.3.1, as well as the latest master (2.4.0-SNAPSHOT).

Start with modules/swagger-codegen/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache and inspect its serialize method. Compare it with modules/swagger-codegen/src/main/resources/Java/ApiClient.mustache and the other Java library templates mentioned in the issue. Done means the generated okhttp-gson Java client supports a String POST body with a text/plain media type instead of raising the unsupported-content-type exception.