[FIX] jQuery.sap.includeStyleSheet: prevent tag replacement with same
href

- moved callback checks to the left to prevent unnecessary URI checks
- use full URI to check against the stylesheet's "href" attribute (which
is always the full URI)
- refactored surrounding if/else
- fixed typo in log message

Change-Id: I11352b8b29aafcb295a3a22e44e1417e412c2b0b