compatible with generator function

[iamnotstone]

In some times the function can be a generator. So have to write the set function in this way:

set(key, value){
  if(typeof value.next === 'function'){                                       
    let result                                                                
     result = value.next()                                                     
     while(!result.done){                                                      
       result = value.next()                                                   
     }                                                                         
     return store[key] = result.value                                                 
  }                                                                           
  else                                                                        
     return store[key] = value
}

to enable this, I have to make some difference in source code. hope this is helpful!

[codecov-io]

Codecov Report

Merging #75 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #75   +/-   ##
=======================================
  Coverage   97.29%   97.29%           
=======================================
  Files           1        1           
  Lines          37       37           
  Branches        8       10    +2     
=======================================
  Hits           36       36           
  Misses          1        1

Impacted Files	Coverage Δ
src/index.js	97.29% <100%> (ø)	⬆️

[caiogondim]

Could you push some tests for it as well?

[iamnotstone]

test code added