don't coerce header value to string on set

[tinovyatkin]

Official Koa documentation about response.set says This delegates to setHeader which sets or updates headers, but setHeader doesn't convert header value types until sending them to the network. This PR fixes behavior of response.set to be according to the documentation.
Also removes recursion on .set calling .setHeader directly, for performance reason.

[codecov]

Codecov Report

Merging #1394 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1394      +/-   ##
==========================================
- Coverage   99.38%   99.38%   -0.01%     
==========================================
  Files           4        4              
  Lines         485      484       -1     
  Branches      132      130       -2     
==========================================
- Hits          482      481       -1     
  Partials        3        3              

Impacted Files	Coverage Δ
lib/response.js	99.36% <100.00%> (-0.01%)	⬇️

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[dead-horse]

This change is reasonable but may contains breaking change, I'd like to merge it when we start next major version's development.

[tinovyatkin]

@dead-horse remember that in current implementation you have two more documented ways to set a response header:

ctx.res.setHeader('Content-Length', 256);
ctx.response.header['content-length'] = 256;

I've linked above that official documentation clearly states that ctx.response.set delegates to ctx.res.setHeader while in fact, it's not exactly the same. So, I don't understand why do you think it's a breaking change while, according to official documentation of Koa, it should be a bug fix.

[dougwilson]

The current major has been out and in use long enough that at this point the bug is in the documentation for it. Seems reasonable to adjust it in next major as @dead-horse said.

[anlexN]

good idea.
@dead-horse can we make koa v3 branch to develop?
@tinovyatkin can we together do it?