Replace gsub with rstrip (#1095)

The regular expression ``/\s*\z/`` matches any whitespace characters at end of string, which means only one match is possible with ``gsub``, replacing it with an empty string. The same can be achieved by ``String#rstrip``.

Author: Maumagnaguagno

lib/irb/completion.rb

@@ -159,7 +159,7 @@ def complete_require_path(target, preposing, postposing)
       else
         return nil # It's not String literal
       end
-      tokens = RubyLex.ripper_lex_without_warning(preposing.gsub(/\s*\z/, ''))
+      tokens = RubyLex.ripper_lex_without_warning(preposing.rstrip)
       tok = nil
       tokens.reverse_each do |t|
         unless [:on_lparen, :on_sp, :on_ignored_sp, :on_nl, :on_ignored_nl, :on_comment].include?(t.event)