Merge branch 'aw/validate-proxy-url-scheme'

Misspelt proxy URL (e.g., httt://...) did not trigger any warning
or failure, which has been corrected.

* aw/validate-proxy-url-scheme:
  http: reject unsupported proxy URL schemes

diff --cc http.c
Simple merge

diff --cc t/t5564-http-proxy.sh
Simple merge