Server does not tolerate attributes string without spaces

Exploding on `', '` requires a space in the attribute string (https://github.com/dflydev/dflydev-hawk/tree/master/src/Dflydev/Hawk/Header/HeaderParser.php#L15) which deviates from the regex used in the official HAWK implementation (https://github.com/hueniverse/hawk/blob/master/lib/utils.js#L125).  

Maybe switch to splitting on `,` and doing a `trim()`? 
