Check why duplicated headers can't be send

[fabiobento512]

Whole story here:
#15 (comment)

[fabiobento512]

Seems that the link that added first (https://stackoverflow.com/a/4371395) was related to server responses, which is not a problem for FRequest.

I found this one that specific mentions requests with duplicated headers:
https://stackoverflow.com/questions/36439308/parse-duplicate-http-headers-from-request-in-node

And by the HTTP RFC2616 this only allowed if only a single key is used. So that is also not a problem in FRequest. In this case FRequest should allow the users to override global headers with specific request headers (with the same key name). So I will add that feature back.