Language and encoding support for httpclient

From the jersey docs

Any variant-related HTTP headers previously set (namely {@code Content-Type},
     *                     {@code Content-Language} and {@code Content-Encoding}) will be overwritten using
     *                     the entity variant information.

As such this adds support for making posts etc with variant information such as language. No changes made to existing methods just new methods added for the variant support

Signed-off-by: Robin Smith <robin.smith@clicktravel.com>