Hi David,

Thanks for looking into this.

IMHO it would be very helpful for code debugging, if "add_header" rejects invalid header arguments ad hoc.
As an alternative, the documentation I used for the implementation [https://docs.python.org/3.6/library/urllib.request.html] could be updated.


"Content-Type:: application/x-www-form-urlencoded;charset=utf-8"
A scope resolution operator thingy in the header? - What could possibly go wrong?  ^^