improve json content type detection

The problem was: Comparing with a hardcoded string is bad because
all the letters can either be uppercase or lowercase or mixed from
a Rest Server. So to check if with a regular expression is more elegant
and reliable.
Merge pull request #10 from wwwdata/master

Registered methods errorHandling
Merge branch 'master' of https://github.com/wwwdata/node-rest-client
Merge remote-tracking branch 'source/master'
support HTTP PATCH Method

see: http://tools.ietf.org/html/rfc5789