added setMemcacheInstance method and improved tests code coverage
- fixed incorrect compiling of key in Phalcon\Session\Adapter\Memcache::destroy method (commit aa82fab9b7c6a63bf3c4161c92bdbc4f86d1b94a)
- added Phalcon\Session\Adapter\Memcache::setMemcacheInstance method
- raised Phalcon\Session\Adapter\Memcache tests code coverage to 100%
lowercased null (codesniffer)
Merge branch 'feature/improve-session-memcache-adapter' into 1.3.0