Fix http authentication with proxy

[rramthun]

*) fixes #4195
*) resolve discrepancies between Hop documentation and behaviour w.r.t. empty/null usernames and password in case of HTTP basic auth

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[rramthun]

@hansva do you have any questions or wishes for changes w.r.t. this pull request?

[hansva]

@rramthun It looks good on first sight, just haven't had the time to test yet. Will get to it shortly.

[hansva]

Thanks for the work @rramthun !