Merge branch 'bc/http-empty-auth'

Merge branch 'bc/http-empty-auth'

Some authentication methods do not need username or password, but
libcurl needs some hint that it needs to perform authentication.
Supplying an empty username and password string is a valid way to
do so, but you can set the http.[<url>.]emptyAuth configuration
variable to achieve the same, if you find it cleaner.

* bc/http-empty-auth:
  http: add option to try authentication without username