OAuth signature calculator with userAuth#key check

- fixed 2 possible programming accidents:
  - added return in a switch/case logic (ProxyUtils)
  - a assignment instead of boolean comparing in RFC2616DateParser

- added a check if userAuth.key is null in OAuthSignatureCalculator, now this class can also be used to request access token in the OAuth1 dance (we can use this logic in the Evernote OAuth process now)