Normalize http.response.status_code

[ribice]

This PR adds:

http method/response code to Echo and net/http
http response code to fasthttp and gin

Adds transaction tests for net/http

Once other integrations are covered in #809, #808 and #807 - all of them should have http.response.status_code which I would assume would resolve #667 .

[codecov]

Codecov Report

Attention: Patch coverage is 89.65517% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 81.09%. Comparing base (9311a06) to head (96cb304).

Files	Patch %	Lines
http/sentryhttp.go	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
+ Coverage   81.08%   81.09%   +0.01%     
==========================================
  Files          49       49              
  Lines        4087     4100      +13     
==========================================
+ Hits         3314     3325      +11     
- Misses        632      634       +2     
  Partials      141      141              

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