Fix for #1334

[noamt]

Commit the response code only when actually starting to write the payload

[codecov]

Codecov Report

Merging #1335 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1335      +/-   ##
==========================================
+ Coverage   84.34%   84.35%   +0.01%     
==========================================
  Files          27       27              
  Lines        2012     2014       +2     
==========================================
+ Hits         1697     1699       +2     
  Misses        205      205              
  Partials      110      110

Impacted Files	Coverage Δ
response.go	71.05% <100%> (+1.6%)	⬆️
context.go	90.71% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d2c33a...f45968e. Read the comment docs.

[vishr]

@noamt Sorry for the delay. Few points:

• Lets cover all type of response e.g. XML etc.
• I think we should be ok without checking for status == 0 in WriteHeader function
• You don't have to write test for all type of responses, the one you have written is good. Just remove JSON word from the test function name so it is generic.

[noamt]

@vishr I don't see any use of applying this to XML or any other of the methods, as this affects only json.
In XML for example, the XML header is written before the body has been encoded:

echo/context.go

Lines 484 to 487 in e2671fe

	if _, err = c.response.Write([]byte(xml.Header)); err != nil {
	return
	}
	return enc.Encode(i)

XML encoding errors can't be reflected to the user in the current situation.