Set the MIME type BEFORE the payload

When we set the payload, it is converted to bytes using the response's
character set. Changing the MIME type later has no effect on the conversion
to bytes, even though it does change the returned MIME type. This results in
bytes that were encoded using one character set while the response reports a
different character set.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196258752

javatests/google/registry/testing/FakeResponse.java

@@ -60,6 +60,7 @@ public final class FakeResponse implements Response {
 
   @Override
   public void setContentType(MediaType contentType) {
+    checkArgument(payload.isEmpty(), "setContentType must be called before setPayload");
     this.contentType = checkNotNull(contentType);
   }