Fix #282 with error "foreign_keys is not implemented"

lib/annotate/annotate_models.rb

@@ -288,7 +288,9 @@ module AnnotateModels
         fk_info = "#\n# Foreign Keys\n#\n"
       end
 
-      foreign_keys = klass.connection.respond_to?(:foreign_keys) ? klass.connection.foreign_keys(klass.table_name) : []
+      return "" unless klass.connection.supports_foreign_keys? && klass.connection.respond_to?(:foreign_keys)
+
+      foreign_keys = klass.connection.foreign_keys(klass.table_name)
       return "" if foreign_keys.empty?
 
       max_size = foreign_keys.collect{|fk| fk.name.size}.max + 1

spec/annotate/annotate_models_spec.rb

@@ -17,6 +17,7 @@ describe AnnotateModels do
     double("Conn",
       :indexes      => indexes,
       :foreign_keys => foreign_keys,
+      :supports_foreign_keys? => true,
     )
   end