Fixed issue with header function evaluation + added 2 tests

[alekbarszczewski]

Fixes #454

1. Reply header functions were evaluated twice per request.
2. Reply header functions were overridden by evaluated values.

After fix each header function is evaluated only once per request.
After fix each header function is evaluated on each and every request separately.

[alekbarszczewski]

Hmm, the problem with coverage is here: https://coveralls.io/builds/4824108/source?filename=lib%2Frequest_overrider.js#L461. I've coded it to be sure that both response.headers and response.rawHeaders will be evaluated. But it seems that interceptor.headers are identical to interceptor.rawHeaders, but in array (instead of object) representation. Is that true? If yes then I can change:

if (typeof value === "function") {
  // Check if header has not been already evaluated. Evaluate it otherwise.
  if (evaluatedHeaders.hasOwnProperty(key)) {
    response.rawHeaders[rawHeaderIndex + 1] = evaluatedHeaders[key]
  } else {
    response.rawHeaders[rawHeaderIndex + 1] = value(req, response, responseBody);
  }
}

to

if (typeof value === "function") {
  response.rawHeaders[rawHeaderIndex + 1] = evaluatedHeaders[key];
}

[pgte]

@alekbarszczewski agree, that should be enough.

[alekbarszczewski]

@pgte Done, squashed additional commit, thanks.

[pgte]

@alekbarszczewski thanks!

Landed on v6.0.1.

[lock]

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