Tentative fix for #77 and #154

Tentative fix for #77 and #154. The default behaviour ("Accept-Encoding: gzip,deflate") is kept to avoid breaking backward compatibility.
The only problem I see with this fix is that it uses an internal type (com.jayway.restassured.internal.http.ContentEncoding) in the public API of RequestSpecification, but this problem already exists for ObjectMapperType. Maybe this type could be moved outside the internal package ? (I would like to avoid creating a new public "ContentEncoding" that would be a mere copy-paste of the original one)