[fix] Corrected the service mocks for testing to respond with a 304 to values of If-Modified-Since that match Last-Modified

The HTTP spec suggests that clients supply the value of Last-Modified that they previously received from the server to If-Modified-Since. When comparing If-Modified-Since > Last-Modified, however, the Mock object would fail to return a 304 for such a case.
Merge pull request #1955 from concordia-publishing-house/fix-if-modified-since

[fix] Corrected the service mocks for testing to respond with a 304 to values of If-Modified-Since that match Last-Modified