feat: pass some headers to child requests

[dkarlovi]

Closes #5.

[dkarlovi]

@dunglas would appreciate your feedback too.

[darkweak]

https://github.com/darkweak/go-esi/pull/9/files#diff-49119e92a6bdccc63d27f7f553246cf3297eb16dc4a8cc08fa2ab344351facacR91
Apply these changes below:

	if (err != nil || response.StatusCode >= 400) && i.alt != "" {
		rq, _ = http.NewRequest(http.MethodGet, sanitizeURL(i.alt, req.URL), nil)
		addHeaders(headersSafe, req, rq)
		if rq.URL.Scheme == req.URL.Scheme && rq.URL.Host == req.URL.Host {
			addHeaders(headersUnsafe, req, rq)
		}
		response, err = client.Do(rq)

		if err != nil || response.StatusCode >= 400 {
			return nil, len(b)
		}
	}

	if response == nil {
		return nil, i.length
	}

[dkarlovi]

Closing here so not to cause conflicts in #10, see #10 (comment)