Add ability to set the TableDefinition class.

The table definition class can be set via passing
in `:table_definition_class_name` as a connection
parameter.

This is specifically useful when the default 
table definition does not support the column types
in the schema.rb. Specifically, passing in
`ActiveRecord::ConnectionAdapters::PostgreSQL::TableDefinition`
allows column types such as `json` to exist in the
schema.

lib/active_record/connection_adapters/nulldb_adapter/core.rb

@@ -15,6 +15,8 @@ def self.insinuate_into_spec(config)
   # Recognized options:
   #
   # [+:schema+] path to the schema file, relative to Rails.root
+  # [+:table_definition_class_name+] table definition class
+  # (e.g. ActiveRecord::ConnectionAdapters::PostgreSQL::TableDefinition for Postgres) or nil.
   def initialize(config={})
     @log            = StringIO.new
     @logger         = Logger.new(@log)
@@ -25,6 +27,12 @@ def initialize(config={})
     @config         = config.merge(:adapter => :nulldb)
     super *initialize_args
     @visitor ||= Arel::Visitors::ToSql.new self if defined?(Arel::Visitors::ToSql)
+
+    if config[:table_definition_class_name]
+      ActiveRecord::ConnectionAdapters::NullDBAdapter.send(:remove_const, 'TableDefinition')
+      ActiveRecord::ConnectionAdapters::NullDBAdapter.const_set('TableDefinition',
+        self.class.const_get(config[:table_definition_class_name]))
+    end
   end
 
   # A log of every statement that has been "executed" by this connection adapter