Fix "Refused to set unsafe header" in browser by using message-headers to omit standard HTTP RFC headers from being set?

[niftylettuce]

I have created a package message-headers at https://github.com/niftylettuce/message-headers#message-headers that supplies us with standard headers that superagent should not modify if we are in window (browser) environments. Users experience errors like this https://stackoverflow.com/questions/50364832/refused-to-set-unsafe-header-access-control-request-headers-angular-4/50366340 if they attempt to set standard headers that the browser itself is setting.
Perhaps if one of the messageHeader.standard headers is passed, we should omit it (e.g. using a similar approach to Lodash's omit method).