[ruby/uri] Revert "Prevent a warning: URI::REGEXP is obsolete

(https://github.com/ruby/uri/pull/138)"

This reverts commit https://github.com/ruby/uri/commit/c00726a20a00.

https://github.com/ruby/uri/commit/22f5a7a790

test/uri/test_common.rb

@@ -11,7 +11,7 @@ class URI::TestCommon < Test::Unit::TestCase
   end
 
   class Foo
-    include URI::RFC2396_REGEXP::PATTERN
+    include URI::REGEXP::PATTERN
   end
 
   def test_fallback_constants