Saved Responses for 204 No Content Returned as 200s

- [X] I have tried with the latest version of OHHTTPStubs
- [X] I have read the [README](https://github.com/AliSoftware/OHHTTPStubs/blob/master/README.md)
- [X] I have read the [Using the right Swift Version of `OHHTTPStubs` for your project](https://github.com/AliSoftware/OHHTTPStubs#using-the-right-swift-version-of-ohhttpstubs-for-your-project) section
- [X] I have searched in the [existing issues](https://github.com/AliSoftware/OHHTTPStubs/issues?utf8=✓&q=is%3Aissue)
- [X] I have read [the OHHTTPStubs wiki](https://github.com/AliSoftware/OHHTTPStubs/wiki) to see if there wasn't a detailed page talking about my issue

### Environment

- version of OHHTTPStubs: 6.1.0
- integration method you are using:
* [X] Cocoapods
* [ ] Carthage
* [ ] submodule
* [ ] other
- version of the tool you use: CocoaPods 1.6, Xcode Version 10.1 (10B61)

### Issue Description

In my tests, I have some stubbed network responses for when the server returns a 204 No Content response. I have them saved as `.response` files in a test bundle, just like any other response. When they are returned by OHHTTPStubs, however, they’re returned with a 200 status code, with the contents of the file as the data. This is due to the fact that `CFHTTPMessageIsHeaderComplete()` returns `false` with the following response:

```
HTTP/1.1 204 No Content
Date: Sat, 19 Jan 2019 15:13:01 GMT
Connection: close
```

This parsing happens here: https://github.com/AliSoftware/OHHTTPStubs/blob/826a9217fcd468220d154ed0075cbc1a20d0c5c9/OHHTTPStubs/Sources/HTTPMessage/OHHTTPStubsResponse%2BHTTPMessage.m#L48

I can manually create these responses for now, but I’m wondering if we should get the status code from the response before calling `CFHTTPMessageIsHeaderComplete()` to check for 204 status codes, for which we’d return `nil` for the data.