refactor: use absolute equal for string comparison

[ulivz]

Summary

It's recommended to use === instead of == for value comparison. in koa, now null and undefined are considered the same, that makes sense, but not for strings.

[codecov]

Codecov Report

Merging #1251 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1251   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files           5       5           
  Lines         391     391           
======================================
  Hits          391     391

Impacted Files	Coverage Δ
lib/response.js	100% <100%> (ø)	⬆️
lib/request.js	100% <100%> (ø)	⬆️
lib/context.js	100% <100%> (ø)	⬆️
lib/application.js	100% <100%> (ø)	⬆️

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[fl0w]

I don't understand why there's endless need to change functional code that makes use of defined, well documented, coercion 😆.

I'm -1 on this as I see this equivalent to stylistic PRs which are denied by convention previously.

[medanat]

I don't understand why there's endless need to change functional code that makes use of defined, well documented, coercion 😆.

I'm -1 on this as I see this equivalent to stylistic PRs which are denied by convention previously.

#1225 (comment)

[anlexN]

i think it is ok, please merge. when to merge?

[3imed-jaberi]

Thank you @ulivz..
I don't like to be this person but I think should close this PR because it's outdated.
Without talking about documentation, the rest of the code was updated as this PR do in the current Koa version.

@dead-horse @niftylettuce @miwnwski @jonathanong ...