Bugfix: Make setPoster work on mobile devices with native controls

[lucash]

This fixes #2418.
If an mobile device is used and mediaelement is configured to use native
controls in this case, t.container is not defined. Therefore the
setPoster() method threw an error.
In that case now the poster attribute is used, which is the native way
of showing the poster image.

[coveralls]

Coverage remained the same at 73.832% when pulling 7d4781b on lucash:fix-ios-native-controls-setposter into be2e7e8 on mediaelement:master.

[lucash]

@rafa8626
This potentially conflicts with #2312. As I wasn't able to reproduce that issue I was not able to make sure that it is not a problem. Regarding the comment in code, removing the attribute helps if you need to support iOS 3.x - maybe it is needed to suppress setting the attribute for that iOS version? I think using a poster image should not be avoided at all if an iOS device is used with native controls as current iOS versions seem to support it.

[rafa8626]

@lucash Thanks for your PR. At this point, we only support iOS 8 and and above, so the comment for that is invalid

[rafa8626]

I'll review your PR as soon as I can, unless you are gonna modify it based on your comment above. Let me know if it's ready to review. Thanks

[lucash]

Please review. As I'm not able to reproduce above issue I won't change anything.

[rafa8626]

PR merged. Thanks and please star the project to support it