Modules: added headers option to r.subrequest().

The option allows JavaScript code to pass custom request headers
to subrequests:

    r.subrequest('/api', {headers: {'X-Token': secret}})

The option accepts a JavaScript object whose keys and values are
added as request headers to the subrequest.  Parent request headers
are inherited by default; user-supplied headers override matching
parent headers using case-insensitive comparison.

Transport-level headers (Connection, Content-Length,
Transfer-Encoding, Expect) supplied by the caller are silently
ignored.  When a body is provided, Content-Length is derived from
the actual body size regardless of what the caller passes.

Author: xeioex