The APICache now properly updates entries when the Last-Modifed

and/or ETag headers change and the Vary header does not.

Previously, the entries would only be updated if the Vary header

was updated.
Update the MockUrlOpener test harness to compare the provided

If-Modified-Since header against the Last-Modified header.
Added a test for the Cache-Control: no-cacche and Last-Modified

header combination. It makes additional requests after the

Last-Modified header is modified to ensure that the cache entry is

updated correctly.
Updated the test for the Cache-Control: no-cache and ETag header

combination to make another request after the ETag is modified to

verify that the cache entry is updated.

Ran unit tests.