Headers from different sources aren't treated equally

[Fishbowler]

Headers added via addHeader or via request settings in config() are case-normalised.
Headers added via request params aren't.

That this is possible isn't actually documented - and it looks like all request params are supported.

This leads to a larger question:
Should we keep consistency (so in this case, move all normalisation to this block, perhaps), or do we respect the original intent of the developer?

[Fishbowler]

Here's where the config() headers get routed through addHeader so that they're case normalised