[Shell] Requests not shown if 'Content-Type' header missing from request

[philliphoff]

Even if requests are published to the Glimpse Server by the Agent, the Client may not show them if those request message (e.g. web-response) payloads are missing the Content-Type header, as the code assumes its present and throws an exception if not.

This was found due to a separate Agent issue where response headers may not be captured if low-level HTTP APIs are used rather than higher level HTTP or Express APIs.