numeric array keys lost where array is returned over mnet.

Where the xmlrpc method course_enrolments is called, and the relevant course contains users whose username is numeric (eg 99013362), the caller cannot properly extract the username from the resulting array. The affected usernames are renumbered - 0,1,2,3.....

Non-numeric usernames are unaffected.

To reproduce:
set up 2 moodles, get them both trusting each other, and allowing one another to enrol students in their courses. Create 2 users on moodle A "Fred", and "99013362", attempt to enrol Fred and 99013362 in a course shared by moodle B using the networking / enrolment interface. Observe that only fred appears on the LHS of the mnet enrolment interface.