Fix request raw_post_body.rewind in actionpack-4.0.0/lib/action_dispatch/http/request.rb

Current code is calling rewind after assigning raw_post data to return value, then rewinding the data body, so the rewinded data is not getting returned nor setting the @env['RAW_POST_DATA'] variable. This fix rewinds the data body before assigning it to the variable that is returned, which causes the variable to be assigned a valid value.

Closes #11345