Fix iframe error response handling on IE browsers

Somehow if iframe request responeded with standard 4xx or 5xx status
then iframe content is not accesible and "access error" thrown. However
on non standard error codes iframe responds 200 and correct WS response
data. Fixing this by returning `undefined` response. However since it is
WS specific code user must handle this case himself in success event
listener.
Merge pull request #159 from edzius/master

Fix iframe error response handling on IE browsers