Force closing keep-alive connections on old cURL

[rmccue]

cURL <7.22 seems to hold the request open if the connection is
keep-alive and the buffer hasn't been filled. Changing to single
connections is slightly less efficient (although unnoticable in most
uses), but should fix this issue.

If you need keep-alive, set the Connection header manually.

[codecov-io]

Current coverage is 92.25%

Merging #211 into master will increase coverage by +<.01%

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+ Hits           1629       1631     +2   
  Misses          137        137          
  Partials          0          0          

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[schlessera]

@rmccue Do you remember any additional context here around the specific version number you've identified?

I'm asking because actively making use of that specific version for conditional code ended up breaking a lot of things. See #838 & https://core.trac.wordpress.org/ticket/59842

[rmccue]

@schlessera Don't recall the context off the top of my head.

I checked the cURL changelog for 7.22 and not seeing anything super relevant around keepalives, best I can see is:

when sending a request and an error is received before the (entire) request body is sent, stop sending the request and close the connection after having received the entire response. This is equally true if an Expect: 100-continue header was used.
fixed several memory leaks in OOM situations
bad expire(0) caused multi_socket API to hang

Looks like 7.21.7 did have this though:

Force connection close for HTTP 200 OK when time condition matched

Other issues I can see that could be relevant are jonashaag/bjoern#51 and this Stack Overflow question which links https://stackoverflow.com/questions/10285700/curl-error-recv-failure-connection-reset-by-peer-php-curl

Sounds like in any case based on #838, keepalive may be more generally still be broken, either with cURL in PHP or with w.org.