Webservices cause header already sent warnings to fill up error_log

Web services attempt to send headers in instances where they've already been sent. Doesn't hurt functionally, but it does cause 4 warnings to goto the error_log each time a web service is called.

Moodle should make a headers_sent() check before sending off the headers.

Replication instructions:
1. Turn up debugging to include PHP warnings.
2. Make a webservice call
3. Notice the warnings that appear in apaches error log.