Ensure memory stream is at position 0 after Body is copied into it

[EricDahlvang]

Fixes: #2762

[coveralls]

Pull Request Test Coverage Report for Build 84109

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 79.036%

---

Files with Coverage Reduction	New Missed Lines	%
/libraries/integration/Microsoft.Bot.Builder.Integration.AspNet.Core/BotMessageHandler.cs	7	0.0%

Totals
Change from base Build 84044:	-0.2%
Covered Lines:	5512
Relevant Lines:	6974

---

💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.5.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.5.3

[tomlm]