open_for_write fails with 500 server error if branch is not given in URL

[hoytak]

Repro:

In [9]: xet_fs = pyxet.XetFS()
   ...: with xet_fs.transaction:
   ...:     with xet_fs.open("xet://hoytak/cp_test_2/data.dat", 'wb') as f:
   ...:         print("writing")
   ...:         f.write(b"00000")
   ...:         print("done")  # Fine
   ...:

writing
done
Failed to commit https://hub.xetsvc.com/hoytak/cp_test_2/data.dat: HTTP status server error (500 Internal Server Error) for url (https://hub.xetsvc.com/api/xet/repos/hoytak/cp_test_2/commits)