Make on(... optional on WebRequest

[christopheranderson]

Is your feature request related to a problem? Please describe.

The WebRequest object has on(... as required, but it is largely optional if you can always promise that body is populated before you invoke adapter.processActivity. Because it is required, Azure Functions request object doesn't work, as is, with botbuilder

Describe the solution you'd like

Make on(... optional on WebRequest

Describe alternatives you've considered

I built a shim layer that handles this, but booooo.

Additional context

Trying to make Azure Functions work nicely.

[enhancement]