Adjust BindParam as needed for AR

We need `value` to have a writer for `StatementCache` to work when
prepared statements are disabled. This is something I'd like to revert
eventually, either by disabling that form of caching in that case or
re-introducing something like the old `Bind` collector.

The addition of `nil?` is to make `IS NULL` be inserted correctly,
similar to what we already do with quoted and casted nodes

lib/arel/nodes/bind_param.rb

@@ -2,7 +2,7 @@
 module Arel
   module Nodes
     class BindParam < Node
-      attr_reader :value
+      attr_accessor :value
 
       def initialize(value)
         @value = value
@@ -13,6 +13,10 @@ module Arel
         other.is_a?(BindParam) &&
           value == other.value
       end
+
+      def nil?
+        value.nil?
+      end
     end
   end
 end