Switch all servlets from PrintWriter to OutputStreamWriter

ServletResponse.getWriter() returns a PrintWriter, which has this
helpful tidbit in the Javadoc:

"Methods in this class never throw I/O exceptions, although some of
its constructors may. The client may inquire as to whether any errors
have occurred by invoking checkError()."

I can count the number of times we call checkError() in the Gitiles
codebase on zero hands, nor do I want to start.

Use the perfectly good OutputStreamWriter class instead.

Change-Id: I7c17c65d6bd657bc8dc41a2ddd7a2bda34a58033

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/RefServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/RefServlet.java
index 4eaf7f3..bf0f75d 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/RefServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/RefServlet.java
@@ -34,7 +34,7 @@
 import org.eclipse.jgit.transport.RefAdvertiser;
 
 import java.io.IOException;
-import java.io.PrintWriter;
+import java.io.Writer;
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
@@ -188,9 +188,9 @@
   }
 
   private static class TextRefAdvertiser extends RefAdvertiser {
-    private final PrintWriter writer;
+    private final Writer writer;
 
-    private TextRefAdvertiser(PrintWriter writer) {
+    private TextRefAdvertiser(Writer writer) {
       this.writer = writer;
     }
 
@@ -201,7 +201,7 @@
 
     @Override
     protected void writeOne(CharSequence line) throws IOException {
-      writer.print(line);
+      writer.append(line);
     }
 
     @Override