Fix creating store credit with amount in foreign format

The numericality validation in rails cannot handle prices
in non-float based amounts (Ie. 100,00 EUR).

Using `Spree::LocalizedNumber` to convert the amount before validation.

core/app/models/spree/store_credit.rb

@@ -40,6 +40,14 @@ class Spree::StoreCredit < Spree::PaymentSource
   extend Spree::DisplayMoney
   money_methods :amount, :amount_used, :amount_authorized
 
+  # Sets this store credit's amount to a new value,
+  # parsing it as a localized number if the new value is a string.
+  #
+  # @param number [String, #to_d] a new amount
+  def amount=(number)
+    self[:amount] = Spree::LocalizedNumber.parse(number)
+  end
+
   def amount_remaining
     return 0.0.to_d if invalidated?
     amount - amount_used - amount_authorized

core/spec/models/spree/store_credit_spec.rb

@@ -155,6 +155,38 @@
     end
   end
 
+  describe "#amount=" do
+    let(:store_credit) { described_class.new(amount: amount) }
+
+    context "with an imperial price format" do
+      let(:amount) { "1,000.50" }
+
+      before do
+        expect(I18n).to receive(:t).with(:'number.currency.format.separator') do
+          "."
+        end
+      end
+
+      it "sets the correct amount" do
+        expect(store_credit.amount).to eq(1000.5)
+      end
+    end
+
+    context "with an european price format" do
+      let(:amount) { "1.000,50" }
+
+      before do
+        expect(I18n).to receive(:t).with(:'number.currency.format.separator') do
+          ","
+        end
+      end
+
+      it "sets the correct amount" do
+        expect(store_credit.amount).to eq(1000.5)
+      end
+    end
+  end
+
   describe "#amount_remaining" do
     context "invalidated" do
       before { allow(store_credit).to receive(:invalidated?) { true } }