Fixed bug where HTTPResponse adds bad content-type header

The fix in 0.11.5 to update the mountpoint_wrapper introduced a bug where "Content-Type: text/html; charset=UTF-8" is added in all my responses even though I'm only sending json and already set the appropriate header. Changing it back fixes the problem.
Merge pull request #446 from chrisgilmerproj/master

Fix content-type header in mounted apps.