Introduce RKStatusCodesOfResponsesWithOptionalBodies

Closes #1685
Check for presence of cached and response ETags. Fixes #1599
Merge pull request #1708 from RestKit/bugfix/nil-etag

Check for presence of cached and response ETags. Fixes #1599
Merge pull request #1686 from RestKit/bugfix/no-content-type

Introduce RKStatusCodesOfResponsesWithOptionalBodies