[fixed] Add missing test and a safety check for ssr

[lbelinsk]

Fixes #580 .
Related to #576.

Changes proposed:

• Added missing uncovered test for the scenario when a user wants to use `ReactModal.setElement("some_element"), i.e. on a string type appElement.
• Added some safety check in settlement method when there is no document due to ssr for example. It actually broke my project which uses ssr. I don't see any tests for ssr, but if for some reasons I missed them, I will be more than happy to add a test for my change.

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed. - Not 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. - Not relevant

[coveralls]

Coverage increased (+0.3%) to 87.703% when pulling 3f1ec6d on lbelinsk:master into 5f92df7 on reactjs:master.

[diasbruno]

@lbelinsk Thank you for having a look on this.

[diasbruno]

Released v3.4.5.

[lbelinsk]

No problem!
Thanks for the quick merge :)