#726: Explicit instead of default "master" ref when creating content

I haven't been able to reproduce the bug in #726, but noticed that `RtContentsITCase` creates on the default ref, which is usually but *not necessarily* `master`. The `canUpdateFileContent()` test, however, *explicitly* specifies that `master` be used. It's possible for the file to be created in one ref and updated in another, which will cause the test to fail.