Every invocation of Convey() now replaces nested action instances to retain closed over variables from the specific invocations of the parent scopes
Merge pull request #199 from m4rw3r/mw/closure_variables

Unique closure variables per Convey invocation
Refactorings
A few more test cases.
Now using `:=` instead of `var` to declare test state.

This is possible because of PR #199  :)
Renamed a test var to reflect it type correctly.

To verify the interface is checked correctly from a struct instance to an interface, http.ResponseWriter was replaced with http.Response but the test var was not renamed.
Merge pull request #200 from gisakulabs/ShouldImplement_fix

Renamed a test var to reflect its type correctly.