Update ModalPortal.js

[jsanchez556]

Fixes #[issue number].

Changes proposed:

- Change the contentHasFocus method, in order to avoid errors in IE

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

Is necessary to change contentHasFocus() to contentHasFocus: function() in order to avoid IE errors

[diasbruno]

There are other PRs that fix this issue (about focus)...but this one can be merged and other PRs can be rebased.