Handle Salesforce API responses containing UTF-8 or ISO-8859-1

The handling of the API response previously blew up when checking the
content of the gzip-decoded response against a regexp, on ruby 1.9+.

The reason is because the content of the response is improperly
interpreted as US-ASCII, even if in UTF-8 or ISO-8859-1.
(see: http://www.salesforce.com/us/developer/docs/api/Content/implementation_considerations.htm#topic-title_international)