De-dupe the requestHandler method in src/request.js.

I renamed it to '_requestHandler' to mark it as 'private' method,
distinct from the public requestHandler users, which appear to be
a possible conflict after concatenation.

The logic for possibly setting the Refer header was also de-duped,
and I added a comment there to note why it's set conditionally.
Update auto-generated files to reflect change to src/request.js