DiffServlet: Set headers in text mode

Change-Id: I9c7d5a9b09411878740c8f9cc7e043bfd372ddda

gitiles-servlet/src/main/java/com/google/gitiles/DiffServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/DiffServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/DiffServlet.java
index e189a4f..cff6eba 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/DiffServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/DiffServlet.java
@@ -38,6 +38,7 @@
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.io.Writer;
 import java.util.Arrays;
 import java.util.Map;
 import java.util.Set;
@@ -137,7 +138,8 @@
         return;
       }
 
-      try (OutputStream out = BaseEncoding.base64().encodingStream(newWriter(res))) {
+      try (Writer writer = startRenderText(req, res);
+          OutputStream out = BaseEncoding.base64().encodingStream(writer)) {
         formatDiff(repo, oldTree, newTree, view.getPathPart(), new DiffFormatter(out));
       }
     } finally {