Add test to verify Handler::File returns correct content-type
Add a default_content_type attribute to the Response object.

Part I in refactoring default_content_type attribute from the Dispatcher
to the Response object. This feels a better fit; the only downside is there
is no back references to the context or app from the response.

Update encode_content() and to_psgi() to add this default_content_type
to the response if no content_type was previously specified.
Remove default_content_type attribute to the Response object.

Moves the logic of what content type to apply by 'default' into the
Response object, resulting in much simplier logic within _dispatch_route.

Big win is that the default content type is not applied until it is actually
required. Ref #550 and #551.
Revert "Update headers:"

With the default content-type not being applied to a response until its needed
(just before content encoding, or before the psgi response is generated), we can
restore the previous functionality of allowing other route handlers and hooks
to set the content-type before Handler::File renderd the static content into the
response.

This reverts commit 8171ae8ddb4d5aeafa2233066e0edbb743bf1014.
A test that ensures send_file content_type is correctly set.
Merge branch 'feature/refactor-default-content-type'