message: feat(request): add `Request::body_mut()` method
diff: @@ -58,6 +58,10 @@ impl<B> Request<B> {
  #[inline]
  pub fn body_ref(&self) -> Option<&B> { self.body.as_ref() }
  + /// Get a mutable reference to the Request body.
  + #[inline]
  + pub fn body_mut(&mut self) -> &mut Option<B> { &mut self.body }
  +
  #[doc(hidden)]
  #[inline]
  #[deprecated(since="0.11.12", note="This method will be gone in future versions.")]
  