Bugfix: Not-modified-since condition corrected.

Date comparison condition is inverted.
In particular equal dates produce 304 response.
Updated tests.
Conservative fix for header names comparison.
Merge pull request #49 from PetrGlad/50b3611027b686f88c41cd0d34ddaca363b65917

Corrected date comparison between If-Modified-Since and Last-Modified header values.