Fix Protobuf support - HTTP headers already written

Prior to this commit, the `ProtobufHttpMessageConverter` would call
`outputMessage.getBody()` at the beginning of the `writeInternal`
method, thus writing HTTP headers. Since this method is trying to write
"x-protobuf" headers after that, protobuf support wasn't working
properly for the default "x-protobuf" media type.

Thanks Toshiaki Maki for the repro project!

Also fixed:
* improve `MockHttpOutputMessage` behavior to reproduce the read-only
state of HTTP headers once they've been written.

Issue: SPR-12229

spring-web/src/test/java/org/springframework/http/MockHttpOutputMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2014 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,17 +32,21 @@ public class MockHttpOutputMessage implements HttpOutputMessage {
 
 	private final ByteArrayOutputStream body = spy(new ByteArrayOutputStream());
 
+	private boolean headersWritten = false;
+
 	@Override
 	public HttpHeaders getHeaders() {
-		return headers;
+		return (this.headersWritten ? HttpHeaders.readOnlyHttpHeaders(this.headers) : this.headers);
 	}
 
 	@Override
 	public OutputStream getBody() throws IOException {
+		this.headersWritten = true;
 		return body;
 	}
 
 	public byte[] getBodyAsBytes() {
+		this.headersWritten = true;
 		return body.toByteArray();
 	}