Expose bug with response codes

[bmarini]

This is related to the 204 no content changes made #1532 and #1549 #1550

Seems like the behavior should be as follows:

1. If status was not set: use that.
2. If status was not set:
    a) If body is formatted to an empty string, use 204 No Content
    b) If body is formatted to a non empty string, use a default status for method, (GET: 200, POST: 201, PATCH: 200, DELETE: 200). 

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

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📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#1565](https://github.com/ruby-grape/grape/pull/1565): Expose bug with response codes - [@bmarini](https://github.com/bmarini).

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[bmarini]

I ran into some issues trying to fix at the status method in inside_route.rb:

      def status(status = nil)
        case status
        when Symbol
          raise ArgumentError, "Status code :#{status} is invalid." unless Rack::Utils::SYMBOL_TO_STATUS_CODE.keys.include?(status)
          @status = Rack::Utils.status_code(status)
        when Integer
          @status = status
        when nil
          return @status if @status
          case request.request_method.to_s.upcase
          when Grape::Http::Headers::POST
            201
          when Grape::Http::Headers::DELETE
            if @body.present?
              200
            else
              204
            end
          else
            200
          end
        else
          raise ArgumentError, 'Status code must be Integer or Symbol.'
        end

1. @Body isn't set as the return value of the endpoint block.
2. If @Body is {} or [] it'll still evaluate as not present, even though it'll serialize to json as a non empty string.

So I think the logic has to be moved til after the formatters run to know if the status should be set to 204 or not.

[coveralls]

Coverage decreased (-0.008%) to 99.063% when pulling ede533e on remind101:expose-response-code-bug into e1a14bf on ruby-grape:master.