Update prompt param if it's already provided in the url (fix fo #636)

[tamerlan]

Extracted the function to add parameter to the url. It should prevent duplicated params to appear.

[msftclas]

All CLA requirements met.

[rohitnarula7176]

@tamerlan @tamerlan Thank you for your contribution. We cannot accept this pull request because your code allows the developers to overwrite the value for prompt in acquireToken methods which we cannot allow by design as it happens in a hidden iframe where prompt cannot be anything other than none. I have made the code change in dev branch such that if you pass prompt value through extraQueryParameter, it will only get added to the login method of adal which is the only one that does not specify prompt value by default. The recommended value for prompt is 'select_account' for login.