Do not send headers by SAPI

[twose]

CC @sshymko

Please pull this branch to verify if it meets your expectations

Close ##3552

[codecov]

Codecov Report

Merging #3571 into master will increase coverage by 0.63%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3571      +/-   ##
==========================================
+ Coverage   45.41%   46.05%   +0.63%     
==========================================
  Files          76       76              
  Lines       14181    14393     +212     
==========================================
+ Hits         6441     6629     +188     
- Misses       7740     7764      +24     

Impacted Files	Coverage Δ
src/os/async_thread.cc	73.33% <0.00%> (+1.33%)	⬆️
src/protocol/base.cc	66.21% <0.00%> (+4.05%)	⬆️
src/coroutine/socket.cc	51.93% <0.00%> (+8.15%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5813a64...c30cd20. Read the comment docs.

[sshymko]

@twose
Could you please add the tests headers_sent.phpt and headers_sent_coroutine.phpt from #3552 to the branch?
If both tests pass, that would indicate the changes meet the expectations.

Thanks for your help!

[twose]

Passed

[sshymko]

@twose
The changes look good and the tests do pass. Thanks!
However, the tests have different names which would make it difficult to maintain them.
Submitted #3574 to group the tests together as they're identical just testing coroutines disabled/enabled.