schema cache already has the columns as a hash, so use that

Commits

  978ec98c8e and
  51676652a3

changed database statements to use the schema_cache methods, added on
master in

  c99e34e90d763c52cbe8dc3d950ed1b4db665dc4 and
  dc973e78560a6514ab172f0ee86dc84a9147d39a

But apparently the methods weren't added to schema_cache, resulting in
the failure described in #8322 for 3-2-stable.

Fixes #8322.

Conflicts:
	activerecord/lib/active_record/connection_adapters/schema_cache.rb

activerecord/lib/active_record/connection_adapters/schema_cache.rb

@@ -1,7 +1,7 @@
 module ActiveRecord
   module ConnectionAdapters
     class SchemaCache
-      attr_reader :columns, :columns_hash, :primary_keys, :tables
+      attr_reader :primary_keys, :tables
       attr_reader :connection
 
       def initialize(conn)
@@ -30,6 +30,25 @@ module ActiveRecord
         @tables[name] = connection.table_exists?(name)
       end
 
+      # Get the columns for a table
+      def columns(table = nil)
+        if table
+          @columns[table]
+        else
+          @columns
+        end
+      end
+
+      # Get the columns for a table as a hash, key is the column name
+      # value is the column object.
+      def columns_hash(table = nil)
+        if table
+          @columns_hash[table]
+        else
+          @columns_hash
+        end
+      end
+
       # Clears out internal caches
       def clear!
         @columns.clear