Subscription parameters wrapped in "headers" attribute

[jasonkatz]

Hi,

I'm looking to pass some auth information in subscription parameters, but the payload for the connection initialization wraps the headers in a nested headers attribute (here, instead of passing them as-is.

This assumes that the target server looks inside a headers property on the incoming parameters - different servers handle these payloads differently. Some added flexibility to the client library will make it much more versatile and usable against various servers.

I will submit a quick PR to address this, but looking forward to hearing any thoughts!