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](https://github.com/undees/rforce/blob/efa0f54a8b4b2588675b31ddde1eba0c4bde0534/lib/rforce/binding.rb#L215), on ruby 1.9+.

The reason is that the content of the response is improperly interpreted as US-ASCII, even if in UTF-8 or ISO-8859-1 ([see salesforce documentation](http://www.salesforce.com/us/developer/docs/api/Content/implementation_considerations.htm#topic-title_international)).