Link preview doesn't work on XHTML pages with Content-Type header as

Link preview doesn't work on XHTML pages with Content-Type header as
‘application/xhtml+xml’
https://bugs.webkit.org/show_bug.cgi?id=150740
-and corresponding-
rdar://problem/23063585

Reviewed by Tim Horton.

For XHTML pages, the element names are lowercase. This patch uses
equalIgnoringCase to fix the bug.
* UIProcess/ios/WKContentViewInteraction.mm:
(-[WKContentView _actionForLongPress]):
(-[WKContentView _interactionShouldBeginFromPreviewItemController:forPosition:]):
(-[WKContentView _dataForPreviewItemController:atPosition:type:]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191830 268f45cc-cd09-0410-ab3c-d52691b4dbfc