Use status instead of statusText

[fuegas]

As of HTTP/2 the field statusText can be empty ( See: jquery/api.jquery.com@be6de73 ). Thus the field statusText is no longer relyable to base logic on. The field status of the XMLHttpRequest remains intact, so we can base the logic on that field.

We encountered this problem during testing of RailsAdmin 3.0 on local development environments versus acceptance environments. On the acceptance environments the success events wouldn't be fired anymore because the statusText was empty and thus the check status == "OK" always returned false.

[coveralls]

Coverage decreased (-5.4%) to 90.547% when pulling deecd07 on fuegas:statustext-http2 into 7301d9b on railsadminteam:master.

[mshibuya]

Looks good, thanks!