Add failing test for Dot visitor and BindParam

Since BindParam has no value, treating it like a string
causes it to fallback to Object#to_s, leading to output like
`#<Arel::Nodes::BindParam:0x007fa43c866108>`.

Since angle brackets are significant in Dot labels, this causes
`Error: bad label format` when passing the graph into dot.

test/visitors/test_dot.rb

@@ -70,6 +70,12 @@ module Arel
           @visitor.accept binary, Collectors::PlainString.new
         end
       end
+
+      def test_Arel_Nodes_BindParam
+        node = Arel::Nodes::BindParam.new
+        collector = Collectors::PlainString.new
+        assert_match '[label="<f0>Arel::Nodes::BindParam"]', @visitor.accept(node, collector).value
+      end
     end
   end
 end