Adding functions to retrieve the contents of the body

Adding functions to retrieve the contents of the body ( if properly encoded ) on a POST ( probably PUT, and PATCH but those are not tested ).

The *string_of_body_exn* function returns the entire body as one string. 

The *list_of_body_exn* returns a value similar to `[(key1, [v1;v2]);(key2, [x1])]`.