fixed the case where a 4xx error would not populate the exception properly and we'd end up with a wrong message

Credit goes to CostaC for his solution described here:
https://github.com/twilio/twilio-php/issues/133#issuecomment-21303014
explicitly set the http_build_query separator

implemented as described:
https://github.com/twilio/twilio-php/issues/139
Merge pull request #142 from caseysoftware/http-build-query-default

Explicitly set the parameter separator for http_build_query