Check if the incoming request contains the "Content-Type" header field, and it contains the give mime type.

Examples:

 // With Content-Type: text/html; charset=utf-8
 req.is('html');
 req.is('text/html');
 req.is('text/*');
 // => true

 // When Content-Type is application/json
 req.is('json');
 req.is('application/json');
 req.is('application/*');
 // => true

 req.is('html');
 // => false