Request and Writer Getters in Context

[SPie]

I added getter functions for the Request and Writer field in Context.
With these changes, it will be easier to create an interface for Context in a project using gin and because of this, easier to mock.

[codecov]

Codecov Report

Merging #2281 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2281   +/-   ##
=======================================
  Coverage   98.38%   98.39%           
=======================================
  Files          41       41           
  Lines        2296     2300    +4     
=======================================
+ Hits         2259     2263    +4     
  Misses         21       21           
  Partials       16       16           

Impacted Files	Coverage Δ
context.go	98.05% <100.00%> (+0.01%)	⬆️

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[appleboy]

@SPie Please give the more example?

[SPie]

@appleboy For example, I use a library with a function which needs a http.Request and because I want to use an Interface for gin.Context I would need to have a an interface function to get the Request out of the Context.