U4-2635 add failing test

Only first occurrence of oldString is replaced.
Fixes: U4-2635 Umbraco.Core.Strings string Rep...

...lace(this string source, string oldString, string newString,
StringComparison stringComparison) method only replaces first ocurrence
of oldString
U4-2635 Skip already-checked part of source input
U4-2635 Skip past replaced word when checking

Avoids infinite loop if search and replacement are equal
Merge pull request #367 from lctjohn/7.1.1-U4-2635

fix for: U4-2635 Replace [...] method only replaces first ocurrence of oldString