Change error type for redirect parsing errors (#1649)

* Change error type for redirect parsing errors

This changes the error type returned when the proxy fails to parse the
redirect target to be a 400 error instead of a 500 error.

As far as I can tell, the only way that this can fail is a failure to
parse the properties of the request to identity the redirect target.
This indicates that the user has sent a malformed request, and so should
result in a 400 rather than a 500.

I've added a test to exercise this, based on a real work example.

* Update changelog

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

oauthproxy.go

@@ -714,7 +714,7 @@ func (p *OAuthProxy) doOAuthStart(rw http.ResponseWriter, req *http.Request, ove
 	appRedirect, err := p.appDirector.GetRedirect(req)
 	if err != nil {
 		logger.Errorf("Error obtaining application redirect: %v", err)
-		p.ErrorPage(rw, req, http.StatusInternalServerError, err.Error())
+		p.ErrorPage(rw, req, http.StatusBadRequest, err.Error())
 		return
 	}