Why does requesting a URL from a Google App Engine frontend with "Connection: close" show an SSLRead() error on Mac OS X

gistfile1.txt

curl -H "Connection: close" -vvv "https://www.khanacademy.org/humanities/art-history-basics" > /dev/null

Transcript:

-> % curl -H "Connection: close" -vvv "https://www.khanacademy.org/humanities/art-history-basics" > /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying 72.14.249.132...
* Connected to www.khanacademy.org (72.14.249.132) port 443 (#0)
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
* Server certificate: *.khanacademy.org
* Server certificate: Go Daddy Secure Certificate Authority - G2
* Server certificate: Go Daddy Root Certificate Authority - G2
> GET /humanities/art-history-basics HTTP/1.1
> Host: www.khanacademy.org
> User-Agent: curl/7.43.0
> Accept: */*
> Connection: close
> 
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0< HTTP/1.1 200 OK
< Content-Type: text/html; charset=utf-8
< Cache-Control: no-cache
< X-Frame-Options: SAMEORIGIN
< X-Content-Type-Options: nosniff
< Vary: Accept-Encoding
< Date: Thu, 21 Apr 2016 01:31:02 GMT
< Server: Google Frontend
< Set-Cookie: fkey=1.0_ubOxjXx-YNPKmA==_1461202261; Path=/
< Set-Cookie: gae_b_id=X2dhZV9iaW5nb19yYW5kb206S3c1V3drUGhhXzZOSE91Sm1ZTDFEU2hOa2tZX3RsaGFHY1dQc2RtcA==; expires=Fri, 21 Apr 2017 01:31:02 GMT; Max-Age=31536000; Path=/
< Set-Cookie: ka_session=5eChFHaOtGDwmEOM30C6qDpkFS-FpT5jW5utJ-9e:1461202261:1461202261; expires=Fri, 21 Apr 2017 01:31:02 GMT; Max-Age=31536000; Path=/
< Alt-Svc: quic=":443"; ma=2592000; v="32,31,30,29,28,27,26,25"
< Expires: Thu, 21 Apr 2016 01:31:02 GMT
< Accept-Ranges: none
< Connection: close
< 
{ [649 bytes data]
* SSLRead() return error -9806
* Closing connection 0
curl: (56) SSLRead() return error -9806