Restrict Parser check to current namespace to avoid conflict with ::Parser

Author: rwstauner

lib/uri/common.rb

@@ -23,7 +23,7 @@ module URI
   Ractor.make_shareable(DEFAULT_PARSER) if defined?(Ractor)
 
   def self.parser=(parser = RFC3986_PARSER)
-    remove_const(:Parser) if defined?(Parser)
+    remove_const(:Parser) if defined?(URI::Parser)
     const_set("Parser", parser.class)
 
     remove_const(:REGEXP) if defined?(REGEXP)