Option to disable content-type guessing by URL

Add a new option to disable content-type guessing
based on the extension of the URL.

Change-Id: I5bea41fcc1011cfb19255cc0333924034007a645
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge pull request #168 from dhellmann/feature/disable-content-type-detection

Option to disable content-type guessing by URL