feat: allow bodyless responses for non empty status codes

[ejose19]

Fixes #1446

Either "Content-Length" must be left (resulting in 0) or we need to add "Connection: close" header as well. The former is what it's in this PR, but opinions are welcomed if the later is better.

If approved will add test cases. Usage:

app.use((ctx, next) => {
  ctx.body = null; // Must be set first, as this sets status to 204 automatically
  ctx.status = 404;
});

[codecov]

Codecov Report

Merging #1447 into master will decrease coverage by 70.24%.
The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           master    #1447       +/-   ##
===========================================
- Coverage   99.37%   29.13%   -70.25%     
===========================================
  Files           4        4               
  Lines         479      484        +5     
  Branches      128      130        +2     
===========================================
- Hits          476      141      -335     
- Misses          0      320      +320     
- Partials        3       23       +20     

Impacted Files	Coverage Δ
lib/response.js	25.15% <0.00%> (-74.21%)	⬇️
lib/application.js	60.68% <75.00%> (-37.55%)	⬇️
lib/request.js	10.49% <0.00%> (-89.51%)	⬇️
lib/context.js	28.26% <0.00%> (-71.74%)	⬇️

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[dead-horse]

I'm OK with this change, can you add a test case for this?

[ejose19]

@dead-horse Done, should be good now.