More robust get constructor name

[lucasfcosta]

Depends on #9.

Due to chaijs/chai#813 tests in this module started failing because of poorly constructed errors (chaijs/chai#45).

This change also improves detection to whether we've got to run the polyfill or not, because if something already has a .name we can get it instantly.

Also, the new if clause inside getConstructorName guarantees we will be using the result of getFunctionName(new errorLike()) only when it's useful, otherwise the empty string is correct, since we're dealing with an anonymous function.

This was the most concise and simplest way me and @vieiralucas could get to a solution.
Also, special thanks to him for pairing with me to solve this 😄

PS.: WORKS ON IE 😎

[vieiralucas]

We really need to extract this into it's own module 😸

[keithamus]

Yeah I feel like if we're going to this much effort to fix this up, let's push it into a separate mini-module. Otherwise though, good changes!

[keithamus]

@lucasfcosta I've created https://github.com/chaijs/get-function-name, if you would like to create a PR to that repo, that'd be swell. Otherwise if I get some time tonight, I'll do it.

[lucasfcosta]

@keithamus Yes! Great idea.
It would be great since it can get pretty annoying to keep things in sync between Chai and the Check-Error repo.
Also, we can avoid introducing errors into a module whenever changing code on the other one.

[lucasfcosta]

I'm moving this there. But let get this merged anyway since it has changes to getConstructorName too.
As soon as that new module gets pushed out we can start using it in check-error and Chai.

EDIT: This is done. Let's merge chaijs/get-func-name#1 and generate saucelabs tokens for release.

[meeber]

LGTM

[vieiralucas]

Can't we merge only #12 and close this?

[meeber]

Yup either way works.

Edit: Yeah, let's close this one and merge the other; the commits don't have the same hash id.