Fix error when building activity with non-string text

[ceciliaavila]

#minor
Related BF-Composer Issue: microsoft/BotFramework-Composer#9692

Description

This PR adds a validation to avoid sending object values as the activity's text and speak.
With this change, we are matching JS implementation.

Specific Changes

• Added a validation for the text and speak properties in the BuildActivity method of the ActivityFactory class.
• Added a unit test to cover this case in ActivityFactoryTests.
• Added a new messageActivity template to use in the new test.

Testing

These images show how the property was displayed before and after the fix.

[coveralls]

Pull Request Test Coverage Report for Build 385156

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 78.391%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Connector.Streaming/Session/StreamingSession.cs	3	90.09%
/libraries/Microsoft.Bot.Builder/ActivityFactory.cs	12	89.1%

Totals
Change from base Build 384727:	0.0%
Covered Lines:	26184
Relevant Lines:	33402

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💛 - Coveralls

[BruceHaley]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll