message: feat(client): add an accessor for the request body
  Allow users to access the body of the request.
  Useful when one wants to modify the request based
  on the data in the body, e.g. to add checksum headers.
diff: @@ -46,6 +46,10 @@ impl<B> Request<B> {
  #[inline]
  pub fn method(&self) -> &Method { &self.method }
  + /// Read the Request body.
  + #[inline]
  + pub fn body(&self) -> Option<&B> { self.body.as_ref() }
  +
  /// Set the Method of this request.
  #[inline]
  pub fn set_method(&mut self, method: Method) { self.method = method; }
  