Call endBackgroundTask at the end of start method

beginBackgroundTaskWithName and endBackgroundTask were not balanced when SDWebImageContinueInBackground option is specified.

According to [Apple's document](https://developer.apple.com/library/ios/documentation/iphone/conceptual/iphoneosprogrammingguide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW20), they must be balanced.