Adding a headers setter to match the request api in the response api

[MYDIH]

Hi,

In a matter of consistency, I wish to be able to use a headers setter on the response, like I would on a request.

Thanks

PS: Koa is amazing, thank you guys

[codecov]

Codecov Report

Merging #1237 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1237   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files           5       5           
  Lines         391     392    +1     
======================================
+ Hits          391     392    +1

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

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[dead-horse]

-0 for me, I'd not like to add a duplicate API, and most of users are already used to set header with response.set(headers)

[fl0w]

I'm +/-0 also.

[slaskis]

I can see how res.headers = is expected usage.

If anything maybe we can freeze the res object from properties that are not in use by koa to avoid confusion? Probably a very breaking change found at runtime sadly.

[MYDIH]

I know that adding a duplicate API seems useless, but there is already one in the Request (headers is just calling header) ... I personnaly think that consistency should be preserved between the two APIs (adding set/get methods to Request is fine too). Having this would make things more obvious ...

[3imed-jaberi]

I'm +/-0 also. But I can say this help the newcomers.