[ruby/uri] Only use UnboundMethod#bind_call if it is available

This allows tests to pass on Ruby 2.4-2.6.

Fixes #19

https://github.com/ruby/uri/commit/67ca99ca87

lib/uri/rfc2396_parser.rb

@@ -322,9 +322,15 @@ module URI
     end
 
     @@to_s = Kernel.instance_method(:to_s)
+    if @@to_s.respond_to?(:bind_call)
       def inspect
         @@to_s.bind_call(self)
       end
+    else
+      def inspect
+        @@to_s.bind(self).call
+      end
+    end
 
     private
 

lib/uri/rfc3986_parser.rb

@@ -79,9 +79,15 @@ module URI
     end
 
     @@to_s = Kernel.instance_method(:to_s)
+    if @@to_s.respond_to?(:bind_call)
       def inspect
         @@to_s.bind_call(self)
       end
+    else
+      def inspect
+        @@to_s.bind(self).call
+      end
+    end
 
     private