Added Header:has method to determine if a header with a provided key exists (case-insensitive). Added a 'useEncoding' argument to the post and update methods of the Curl provider, when sending as application/json and perhaps other content-types, we don't want to urlencode the data.
Tidied up the initPostFields method a little more of the Curl Http\Provider.
Merge pull request #199 from flyingbuddha/master

Phalcon\Http\Client\Provider\Curl can optionally post unencoded data