Add support for proxy-only headers (`destHeaderBlackList`).
Renamed new option to proxyHeaderExclusiveList for clarity.

Updated docs.
Simplify Proxy-Authorization header handling

Deprecate mess with removing and re-adding Proxy-Authorization header in favor of proxyHeaderExclusiveList.
Small refactoring.
Fix linting error.
Fix Travis test.

Not sure why is it "Connection: close" when testing locally.
Merge pull request #1215 from RReverser/master

Add proxyHeaderExclusiveList option for proxy-only headers.