Fix SRP auth response

[3oris]

Align flow to official java SDK

See: https://github.com/aws/aws-sdk-android/blob/master/aws-android-sdk-cognitoidentityprovider/src/main/java/com/amazonaws/mobileconnectors/cognitoidentityprovider/CognitoUser.java#L2914 ff

Fixes #125

[3oris]

Cannot test due to

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions

[mingofmongo]

Hi, @3oris , I can confirm that your fix works! Just tried it out with a client app that has client secret. Internal username (AWS's "sub") and self.username (email) are indeed different.

[bwindsor]

I can also confirm that this fix works. Please could it be merged?

[blueskycorner]

Any update ?
Thanks