fix($httpBackend): compare timeoutId with undefined in completeRequest

httpBackend with ngMock browser.defer can never cancel the first deferredFn because the timeoutId returned by defer for the first fn is a zero value.
Compare timeoutId with undefined fix this issue.
also, please write this as `if (timeoutId !== void 0) $browserDefer.cancel(timeoutId);` --- it's much cleaner this way. (note that we need braces around the block IIRC)