Added logic to dialogs to protect against non-message activities

[Stevenic]

The new OAuthPrompt needs to receive "event" and "invoke" type activities. That means that since we can no-longer expect only "message" activities passed to dialogs we should add some additional protection in a few places.

[coveralls]

Pull Request Test Coverage Report for Build 276

• 7 of 9 (77.78%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.08%) to 83.267%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-dialogs/src/prompts/prompt.ts	2	3	66.67%
libraries/botbuilder-dialogs/src/waterfall.ts	5	6	83.33%

Totals
Change from base Build 270:	-0.08%
Covered Lines:	1556
Relevant Lines:	1816

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💛 - Coveralls

[johnataylor]

And I have these in the C# version. I made those changes on the choice branch so they can go in with that.