Fix URI::RFC2396_PARSER issue with older Rubies

Switching to URI::RFC2396_PARSER in 028fd33 resulted in
problems with older versions of Ruby where URI::RFC2396_PARSER is not
defined. Although URI::RFC2396_PARSER was added and backported to the
vendored uri library in older versions of Ruby, we check for
URI::RFC2396_PARSER's presence such that the warnings are avoided when
using a newer non-vendored version of the gem.

lib/rack/utf8_sanitizer.rb

@@ -256,7 +256,11 @@ def unescape_unreserved(input)
     # Performs the reverse function of `unescape_unreserved`. Unlike
     # the previous function, we can reuse the logic in URI#encode
     def escape_unreserved(input)
-      URI::RFC2396_PARSER.escape(input, UNSAFE)
+      if Object.const_defined?("URI::RFC2396_PARSER")
+        URI::RFC2396_PARSER.escape(input, UNSAFE)
+      else
+        URI::DEFAULT_PARSER.escape(input, UNSAFE)
+      end
     end
 
     def sanitize_string(input)