Add proxyHeaderExclusiveList option for proxy-only headers.

Fixes #1070 by adding `proxyHeaderExclusiveList` for list of headers that should be sent exclusively to proxy in tunnel mode.

This also simplifies handling of `Proxy-Authorization` by avoiding old flow where:
1) header was firstly removed in any case and temporarily stored to inner property;
2) when in tunnel mode, it was added back to `proxyHeaders` hash;
3) when not in tunnel mode, it was added back to regular `headers` where it was firstly removed from.

Now any header from `proxyHeaderExclusiveList` (incl. `Proxy-Authorization`) goes through simpler flow:
1) if in tunnel mode, move it from regular `headers` to `proxyHeaders`;
2) that's it :P

So it covers the same flow that `Proxy-Authorization` had, and at the same time gives ability to extend list with custom proxy-only headers.

Incorporates #1028 and #1050.