Avoid exceptions for request without Content-Type header #1881

[ivopisarovic]

Description

If a non-bot request comes to the server, it raised an exception because some request may not have content-type header. Applies to bots with telemetry middleware.

Specific Changes

• Improved the condition to check if the content-type header is present before checking its value.

Testing

• Nothing new added.

An updated version of the original PR https://github.com/microsoft/botbuilder-python/pull/1881.