fix: --custom-header flag implementation

[consideRatio]

The --custom-header flag added in #206 didn't seem to work, this PR attempts to fix this and also adds a test to make sure.

Given current node-http-server, the key fixes are:

• A singular/plural typo for customHeader(s)
• node-http-proxy configuration API, it now needs to be headers and not customHeaders
• headers now needs to get an object, not a list of objects

Closes #223.

[consideRatio]

@minrk I think this is ready for merge

[consideRatio]

@minrk as the current implementation isn't working, and I've verified this implementation works with tests. I'll go ahead and self-merge this tomorrow.

[consideRatio]

I did a another self review of my code and became more confident the implementation to fix the current broken logic makes sense. I'll go ahead and self-merge this.