Assign new ResponseWriter after calling http.HandlerFunc

[jszwedko]

Otherwise, the http.ResponseWriter passed to next() within the
middleware is unused. This precludes middlewares from wrapping the
http.ResponseWriter to do things like record the status code.

Fixes #1257

There was a workaround mentioned in the linked issue to set the echo.Response.Writer in the middleware, but I think this is still an improvement as:

• the current behavior is, in my opinion, pretty suprising. I expected the rw I passed to next() to get passed to the next middleware. In practice, I can just pass nil here and have the same effect as echo doesn't capture and use the rw
• the mentioned workaround only works if you wrote the middleware. It precludes using 3rd-party HTTP middlewares

[codecov]

Codecov Report

Merging #1341 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   84.39%   84.42%   +0.02%     
==========================================
  Files          27       27              
  Lines        2019     2022       +3     
==========================================
+ Hits         1704     1707       +3     
  Misses        205      205              
  Partials      110      110

Impacted Files	Coverage Δ
context.go	90.79% <100%> (+0.07%)	⬆️
echo.go	85.66% <100%> (+0.04%)	⬆️

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[jszwedko]

I believe this PR to still be relevant. cc/ @vishr

[vishr]

@jszwedko Ignore the bot. I will look into it soon.