Fix compile error on Swift.(This is a workaround)

I try to use AWS iOS SDK(v2) on Swift. (installed via CocoaPods).

```
pod 'AWSiOSSDKv2'
pod 'AWSCognitoSync'
```

But following compile error occur. 
```
/Users/FGtatsuro/workspace/xcode/cognito_sample/Pods/AWSiOSSDKv2/AWSCore/Authentication/AWSSignature.m:598:9: Use of undeclared identifier '_delegate'
```
I think this isn't the fault of AWS SDK, but swift compiler can't identify the name 'delegate' with underscore. (Other underscore variable(ex. _stream) can be identified...)
Thus, I need the workaround to use SDK on Swift.