(Bug 50835) Dont nowiki escape already escaped template params

* In {{echo|foo<nowiki>|</nowiki>bar}}, the pipe-char is already
  safe and doesn't need further escaping.

* New test added to track this fix.

Change-Id: Idc4ad71ba7ce4d2ba48a6a4402ebfa6fd4f76e9c
Merge "(Bug 50835) Dont nowiki escape already escaped template params"