Don't set Content-Type for httperror.Redirect.

Remove workaround for https://groups.google.com/forum/#!topic/golang-nuts/9AVyMP9C8Ac
since it has been fixed upstream in Go 1.10.

code.go

@@ -66,9 +66,6 @@ func (h *codeHandler) ServeCodeMaybe(w http.ResponseWriter, req *http.Request) (
 		if d.Package == nil {
 			u := *req.URL
 			u.Path += "/..."
-			if req.Method == http.MethodGet { // Workaround for https://groups.google.com/forum/#!topic/golang-nuts/9AVyMP9C8Ac.
-				w.Header().Set("Content-Type", "text/html; charset=utf-8")
-			}
 			http.Redirect(w, req, u.String(), http.StatusSeeOther)
 			return true
 		}

httputil/handler.go

@@ -39,9 +39,6 @@ func (h *errorHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 	if err, ok := httperror.IsRedirect(err); ok {
-		if req.Method == http.MethodGet { // Workaround for https://groups.google.com/forum/#!topic/golang-nuts/9AVyMP9C8Ac.
-			w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		}
 		http.Redirect(w, req, err.URL, http.StatusSeeOther)
 		return
 	}

sessions.go

@@ -315,9 +315,6 @@ func (h *sessionsHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 	if err, ok := httperror.IsRedirect(err); ok {
-		if req.Method == http.MethodGet { // Workaround for https://groups.google.com/forum/#!topic/golang-nuts/9AVyMP9C8Ac.
-			w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		}
 		http.Redirect(w, req, err.URL, http.StatusSeeOther)
 		return
 	}
@@ -360,9 +357,6 @@ func (h *sessionsHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 				Path:     "/login",
 				RawQuery: url.Values{returnQueryName: {req.RequestURI}}.Encode(),
 			}).String()
-			if req.Method == http.MethodGet { // Workaround for https://groups.google.com/forum/#!topic/golang-nuts/9AVyMP9C8Ac.
-				w.Header().Set("Content-Type", "text/html; charset=utf-8")
-			}
 			http.Redirect(w, req, loginURL, http.StatusSeeOther)
 			return
 		}