allow setting any header, not just X-headers (issue #66)

[moises-silva]

The idea is good, as I don't think restricting to only X- headers makes sense. However, implementation-wise I'd say this should change to simply accept a sip_headers parameter and let users pass the P-Whatever or X-Whatever and deal internally with the siph prefixing.

If backwards compatibility is a concern (no idea where @tgoodlet stands on that) then keep the current xheaders parameter and add the new sip_headers parameter. Supporting both at least until a major version bump seems like the most sensible option to not break existing use.

[goodboy]

@SteveAyre @moises-silva yes I totally agree. I'd even be partial to just dropping xheaders and just go with headers (like in the requests api) since as @SteveAyre mentioned in #67 that X headers are just a special case (even if a common one). switchy is still in alpha mode so there's not much reason to bother with backward compat as long as the unit tests pass.

I think this is good to land for now :+1:

[vodik]

Thanks for the contribution.