Gateway emulator appears to not work correctly when the server can return GZIP encoded responses

Observed incorrect behaviour when connecting to a Django app that uses the gzip. Making requests using a client that supports gzip content encoding will cause the requests to fail client-side with a content decoding error.

Steps to reproduce:

• Connect API gateway through to an local development API which uses encoding (e.g. the Photo API)
• Make any request via a client which supports GZip encoding (e.g. chrome/most browsers).
• System will return a 200, but content will not be able to be decoded.

Proposed solution: The returned_response needs to have the enable_compression function called before the call to prepare - see aiohttp reference.