Solve http request delay

[hreintke]

Serve next wificlient (http_request) if current client does not have data

Solves #1522
I included also the the proposed change to esp8266/Arduino#8941, just to be sure.

Disclaimer : I did not test with HTTPS, don't have that running in my environment

Edit : This solution is identical to the current implementation of esp8266

Edit 2 : I think the remaining CI failure is not a result of this PR.

[earlephilhower]

Thanks! LGTM after read-thru. I wonder if they updated the aspell version to cause the weird document failure. Let me try fixing that in another PR, merge that change here, then I'll merge this one...