Added fix for incorrect loop in SC.Request.manager.cancelAll

SC.Request.manager.cancelAll was using javascript's internal forEach loop in combination with a method that modified the array over which the loop is operating. The result of this is that every other request was being skipped and not cancelled.

This issue was partially concealed by the fact that the inflight and pending arrays were being overwritten whenever cancelAll was being invoked. This has also been corrected.