Add `is()` method to request and responses

[evert]

Should return true or false if the request or response is of the contenttype specified in is().

Examples that should work if content-type is application/hal+json:

• is('json')
• is('application/json')
• is('application/hal+json')

Only xml and json should be supported for the + syntax.

Bonus feature: would be nice if it supports an array of contenttypes too. If more than 1 are passed, it should return true if any of the contenttypes match.