Bug fix to serialization of interwiki links

* Interwiki prefix was not being normalized properly
  during serialization.
* Updated existing test to cover other variations.
* 5 selser failures are because of separator related
  issues (for which we have a bug report).

Bug: 62743
Bug: 60754
Change-Id: I86024ff038a4d38cdf102ba087533049718b6550