Fix an issue observed in Eclipse / Studio

With the 1.0.4 release of Apigility, you are no longer able to create a new API
via the Apigility interface in Eclipse. It fails silently.

On inspection, we determined that the POST request to create an API was being
sent as a GET request, for no apparent reason. We then started stepping through
commits from zf-apigility-admin's 1.0.2 tag up to the 1.0.3 tag to determine
when this behavior began. The commit that broke was 5c4087c, which was when
cache-control headers were committed. In examining the implementation,
commenting out the Last-Modified header resolved the issues observed in Studio.

Since Last-Modified is the weakest indicator in the set of cache control headers
used, we can safely remove it and still have reasonable expectations that the
cache control requests will be honored.
CS fixes (trailing whitespace)
Merge branch 'hotfix/eclipse-webkit-fix'

Close #224