Fix inconsistent behavior from String#pluralize

Before:
  When calling String#pluralize with count=1 then it returned same
  string, but with count other than 1, returned new string.

After:
  String#pluralize always return a new string.

  => Prevent mutation of a string inadvertently.
Merge pull request #14801 from kuldeepaggarwal/fix-string-inflection

Fix inconsistent behavior from String#pluralize