Add failing test for 304 responses on WCF host

304 Not Modified responses should not have a Content-Type header
Fix WCF host Content-Type issue for 304 responses
Remove inaccurate comment in WCF host

i'm not sure if the "stupid" part is inaccurate, but making the content
type parameter in WCF's `CreateStreamResponse` method blank seems to
keep WCF from sending a Content-Type header in the response.
Use  `string.Empty` instead of `""`
Merge pull request #1365 from jugglingnutcase/wcf-not-modified-header-fix

Keep WCF from sending a `Content-Type` header with 304 Not Modified responses