Remove single space response body for head request

- The single space response was added due to a bug in safari
  in https://github.com/rails/rails/commit/cb0f8fda9652c4d24d04693bdb82cecd3b067e5c
  and
  https://github.com/rails/rails/commit/807df4fcf021fc4d15972aa1c17ba7398d43ab0d.
- This was removed from the `render nothing: true` in
  https://github.com/rails/rails/pull/14883.
 - Removing it from response of :head also. As :head is more obvious
  alternative to call `render nothing:
  true`(http://guides.rubyonrails.org/layouts_and_rendering.html#using-head-to-build-header-only-responses),
  removing it from head method also.
 - Closes #18253.