fix: use typeof operator to check function type

[sobolevn]

closes #99

[sobolevn]

I am pretty sure that

[13:26:34] Requiring external module ts-node/register
SyntaxError: Unexpected token {
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Module.require (module.js:353:17)
    at require (internal/module.js:12:17)
    at Object.<anonymous> (/home/travis/build/typestack/typedi/node_modules/tslint/lib/formatters/codeFrameFormatter.js:20:20)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
npm ERR! Test failed.  See above for more details.
The command "npm test" exited with 1.

is not related to this change.

[justinmchase]

👍

[sobolevn]

@pleerock friendly ping

[github-actions]

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