Fix context leak

[mhr3]

Gorilla sessions leak if the context isn't cleared at the end of a request, this fixes it

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 1.68%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   93.18%   94.87%   +1.68%     
==========================================
  Files           2        2              
  Lines          44       39       -5     
==========================================
- Hits           41       37       -4     
+ Misses          2        1       -1     
  Partials        1        1

Impacted Files	Coverage Δ
session/session.go	100% <100%> (ø)	⬆️
casbin/casbin.go	90% <0%> (+3.04%)	⬆️

---

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 525b19f...5892998. Read the comment docs.

[rskumar]

@vishr please accept this PR. As per gorilla doc, context.Clear should be called to avoid leak. Either to use context.ClearHandler(http.DefaultServeMux) where context is gorilla/context or other way implemented in this PR.

[vishr]

@mhr3 thanks for your contribution 🎉