Fixing bug #34: Idle detection not consistent in IE11

[kazusss]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines: https://github.com/HackedByChinese/ng2-idle/blob/master/CONTRIBUTING.md#pr
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[ ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Coverage remained the same at 100.0% when pulling db89caade36b0fd291c47441da8c89ce9a2ec0e5 on kazusss:fix/34_idle_detection_not_consistent_in_IE11 into 8c7c39c285ef6a3dbf2c45a18e62afcdcdd429f7 on moribvndvs:master.