Extend NumericalityValidator to check precision and scale

Use new checks on Unit.multiplier
Closes #28

lib/core_ext/active_model/validations/numericality_validates_precision_and_scale.rb

@@ -0,0 +1,16 @@
+module CoreExt::ActiveModel::Validations::NumericalityValidatesPrecisionAndScale
+  def validate_each(record, attr_name, value, ...)
+    super(record, attr_name, value, ...)
+
+    if options[:precision] || options[:scale]
+      attr_type = record.class.type_for_attribute(attr_name)
+      value = BigDecimal(value) unless value.is_a? BigDecimal
+      if options[:precision] && (value.precision > attr_type.precision)
+        record.errors.add(attr_name, :precision_exceeded, **filtered_options(attr_type.precision))
+      end
+      if options[:scale] && (value.scale > attr_type.scale)
+        record.errors.add(attr_name, :scale_exceeded, **filtered_options(attr_type.scale))
+      end
+    end
+  end
+end