Accessing the headers of a request, without starting a connection

[AlexBroadbent]

Is there a way to get a map of all the headers in a request without starting the connection?

Currently, calling headers() will begin the connection and so manipulating them in any way after that call will cause an error.

This is causing a real headache, as the only available methods are aimed at specific headers - I want to see all of the headers that will be sent. This is being done through a filter, so I do not have control over how the request is initialised.