explicitly set Content-Type in Recovery handler

writing the panic HTML content without a known content-type header
is unreliable and leads to unpredictable results - particularly when
used with other middleware that sets the header in various cases

recovery.go

@@ -114,13 +114,19 @@ func Recovery() Handler {
 	return func(res http.ResponseWriter, c Context, log *log.Logger) {
 		defer func() {
 			if err := recover(); err != nil {
-				res.WriteHeader(http.StatusInternalServerError)
 				stack := stack(3)
 				log.Printf("PANIC: %s\n%s", err, stack)
 
 				// respond with panic message while in development mode
+				var body []byte
 				if Env == Dev {
-					res.Write([]byte(fmt.Sprintf(panicHtml, err, err, stack)))
+					res.Header().Set("Content-Type", "text/html")
+					body = []byte(fmt.Sprintf(panicHtml, err, err, stack))
+				}
+
+				res.WriteHeader(http.StatusInternalServerError)
+				if nil != body {
+					res.Write(body)
 				}
 			}
 		}()

recovery_test.go

@@ -12,15 +12,19 @@ func Test_Recovery(t *testing.T) {
 	buff := bytes.NewBufferString("")
 	recorder := httptest.NewRecorder()
 
+	setENV(Dev)
 	m := New()
 	// replace log for testing
 	m.Map(log.New(buff, "[martini] ", 0))
+	m.Use(func(res http.ResponseWriter, req *http.Request) {
+		res.Header().Set("Content-Type", "unpredictable")
+	})
 	m.Use(Recovery())
 	m.Use(func(res http.ResponseWriter, req *http.Request) {
 		panic("here is a panic!")
 	})
 	m.ServeHTTP(recorder, (*http.Request)(nil))
 	expect(t, recorder.Code, http.StatusInternalServerError)
+	expect(t, recorder.HeaderMap.Get("Content-Type"), "text/html")
 	refute(t, len(buff.String()), 0)
-
 }