Do not copy Function#length

[ehmicky]

Fixes #13. Previous PR at #18.

This prevents copying Function#length. That property should reflect the actual parameters length, not the one of the function that is being copied.

For example:

const original = value => value
const wrapper = () => original(true)
mimicFn(wrapper, original)
// This should be 0 not 1
console.log(wrapper.length)

[ehmicky]

Fixed.

[sindresorhus]

Do you intend to handle the other issues you opened soon or should I do a new release now?

[ehmicky]

I intend to fix them now. This might be one at a time (to avoid conflicts) depending on the issue though.