Update to allow for specification of form post data charset.

Updates too Connection and Request interfaces and HttpConnection implementation.   This allows for the specification of the charset used for application/x-www-form-urlencoded post data character set, which is needed for more robust support.   The implementation was done by adding a postDataCharset method to the request object with the default value still maintaining the DataUtils.defaultCharset.   This implementation should have minimum effect on current applications as the default is still UTF-8 but allows the setting of ISO-8859-1 which appears frequently in use on the web.  I've also added the charset= parameter to the header setting to allow the server to be aware of the charset used, although I've found this isn't always honored...