Do not set headers if response is already committed

This fixes issues when the response headers were already set and sent
to the client. In all other cases this is a no-op.

Change-Id: Ifb429e78f721cc514bb6eb1a3ef3412425cd2f1c
Signed-off-by: Max Haslbeck <haslbeck@google.com>

org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/GitSmartHttpTools.java

diff --git a/org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/GitSmartHttpTools.java b/org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/GitSmartHttpTools.java
index d2f0cd2..bf3da4b 100644
--- a/org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/GitSmartHttpTools.java
+++ b/org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/GitSmartHttpTools.java
@@ -255,9 +255,11 @@ private static void writePacket(PacketLineOut pckOut, String textForGit)
 	private static void send(HttpServletRequest req, HttpServletResponse res,
 			String type, byte[] buf, int httpStatus) throws IOException {
 		ServletUtils.consumeRequestBody(req);
-		res.setStatus(httpStatus);
-		res.setContentType(type);
-		res.setContentLength(buf.length);
+		if (!res.isCommitted()) {
+			res.setStatus(httpStatus);
+			res.setContentType(type);
+			res.setContentLength(buf.length);
+		}
 		try (OutputStream os = res.getOutputStream()) {
 			os.write(buf);
 		}