Handle string values for variant unit value

The FDC API is returning all values as strings. This should probably be
handled in the Connector long-term.

engines/dfc_provider/app/services/quantitative_value_builder.rb

@@ -33,7 +33,7 @@ class QuantitativeValueBuilder < DfcBuilder
     product.variant_unit = measure
     product.variant_unit_name = unit_name if measure == "items"
     product.variant_unit_scale = unit_scale
-    product.unit_value = quantity.value * unit_scale
+    product.unit_value = quantity.value.to_f * unit_scale
   end
 
   # Map DFC units to OFN fields:

engines/dfc_provider/spec/services/quantitative_value_builder_spec.rb

@@ -104,6 +104,20 @@ RSpec.describe QuantitativeValueBuilder do
       expect(product.unit_value).to eq 0.005
     end
 
+    it "interpretes values given as a string" do
+      quantity = DataFoodConsortium::Connector::QuantitativeValue.new(
+        unit: quantity_unit.KILOGRAM,
+        value: "0.4",
+      )
+
+      builder.apply(quantity, product)
+
+      expect(product.variant_unit).to eq "weight"
+      expect(product.variant_unit_name).to eq nil
+      expect(product.variant_unit_scale).to eq 1_000
+      expect(product.unit_value).to eq 400
+    end
+
     it "knows imperial units" do
       quantity = DataFoodConsortium::Connector::QuantitativeValue.new(
         unit: quantity_unit.POUNDMASS,