text: Title: POSTing to the /redirect-to results in 500 internal server error
  Body: Reproduce either using httpbin's own UI or run `curl -X POST "http://httpbin.org/redirect-to" -H  "accept: text/html" -H  "Content-Type: application/x-www-form-urlencoded" -d "url=http%3A%2F%2Ffoo.bar&status_code=308"`
  
  httpbin responds with
  
  ```
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
  <title>500 Internal Server Error</title>
  <h1>Internal Server Error</h1>
  <p>The server encountered an internal error and was unable to complete your request.  Either the server is overloaded or there is an error in the application.</p>
  ```