Fix doc creation race condition
two doc objects could be created if two clients open the doc simultaneously
when trying to create a document that already exists, run the callbacks with the error 'Document already exists' instead of with the doc itself
Merge branch 'doc-load-race' of https://github.com/obijywk/ShareJS into obijywk-doc-load-race
Merge branch 'obijywk-doc-load-race'
Merge branch 'master' of github.com:josephg/ShareJS