Fixed warnings: lib/rspec/core/metadata.rb:172: warning: Object#=~ is deprecated; it always returns nil (#2582)

Author: walf443

lib/rspec/core/metadata.rb

@@ -169,7 +169,7 @@ def file_path_and_line_number_from(backtrace)
         end
 
         def description_separator(parent_part, child_part)
-          if parent_part.is_a?(Module) && child_part =~ /^(#|::|\.)/
+          if parent_part.is_a?(Module) && /^(?:#|::|\.)/.match(child_part.to_s)
             ''.freeze
           else
             ' '.freeze