Test for both formats of Content-Encoding header

Due to some inconsistencies in how headers are formatted through the code, I added an extra check for 'Content-Encoding' as well as 'content-encoding' when determining if the response should be compressed.

This came up due to wanting to manually gzip a response in a handler, and manually setting the 'Content-Encoding' header. If I used the proper format, the existing code would compress my already compressed response.

I feel this is a band-aid to a bigger issue, that is inconsistent header formatting in what is being checked for and what is being added as headers.

I also added tests for GET requests in the gzip integration test, as well was removed a duplicate test near the end.