Fix custom merge function not working

[Grimones]

Hello

Not sure how no one has noticed that in 2 years but a typeof is missing here

https://github.com/jonschlinkert/set-value/blob/c574eb8d94ba21d22b6fe5a5741fd4dbfc9dc47a/index.js#L115

Regression occurred after https://github.com/jonschlinkert/set-value/commit/cdc74463f5778f2c50b1bae839818ac76268aa51#diff-e727e4bdf3657fd1d798edcd6b099d6e092f8573cba266154583a746bba0f346R115