[fixed] Don't set aria-modal attribute when running on apple's Webkit.

[leonascimento]

Fixes #611.

-This merge request should prevent that the VoiceOver fails.

Changes proposed: -Define false to aria-modal when is webkit.

Upgrade Path:

-Change the value when is webkit.

-isSafariDesktop transforming this function in a helper function to use on others places.

Acceptance Checklist:

• [x] All commits have been squashed to one.
• [x] 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.
• [x] If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage increased (+0.5%) to 90.073% when pulling 287e79f7252d8ebf9a2620fcffb7f159b93e91bb on leonascimento:fix/voice-over-fails into 241b8a697c2141f1631b9666bda07b01f4972f56 on reactjs:master.

[leonascimento]

@diasbruno @MaksimKniazeu I did an update on this pull request. Can you make the code review?

[markerikson]

Uh... I don't maintain this library.

[leonascimento]

@markerikson sorry man, I wrote wrong.