Upgrade to casbin v2 and close #10

[rolandjitsu]

No description provided.

[codecov]

Codecov Report

Merging #11 into master will decrease coverage by 0.19%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master      #11     +/-   ##
=========================================
- Coverage   56.58%   56.39%   -0.2%     
=========================================
  Files           3        3             
  Lines         129      133      +4     
=========================================
+ Hits           73       75      +2     
- Misses         54       55      +1     
- Partials        2        3      +1

Impacted Files	Coverage Δ
casbin/casbin.go	83.33% <66.66%> (-6.67%)	⬇️

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[rolandjitsu]

@vishr how do you want to handle the error that a.Enforcer.Enforce(user, path, method) might return? Return an internal server error to the user?

[hsluoyz]

@rolandjitsu I think it's OK. Maybe you want to check how other middlewares handle errors.