Fix ApiLimitExceedException details, fix for cache saving date
Hard-code sent timezone in If-Modified-Since header

If-Modified-Since format must be RFC850 compliant and in GMT timezone
BUT PHP DATE_RFC850 format keep returning UTC timezone. So, we set the
GMT timezone and force its rendering in the header.
Merge pull request #38 from KnpLabs/bugfix/api_limit

Bugfix/api limit