Fix "document is not defined" error

[diegohaz]

Fixes#834

I see that this was updated on #333. But isUndefined(document) will throw an error if document wasn't set in the first place, while the typeof check will not.

[RobbieTheWagner]

@diegohaz Ah, good catch! I think instead of the changes you proposed, we will want to update the util https://github.com/shipshapecode/tether/blob/master/src/js/utils/type-check.js#L38

It should check if typeof value === 'undefined'. Would you mind updating your PR?

[diegohaz]

@rwwagner90 That will not fix the error as document will still be referenced in isUndefined(document). This isUndefined function can only be used with variables that already have been declared (so it returns true when their value is really undefined or they haven't been assigned a value yet).

It's not specific to document or window. Any variable that hasn't been declared will throw this error:

typeof is a special operator that bypasses this error:

[RobbieTheWagner]

Thanks for the explanation @diegohaz and thanks for the PR!