Shared money input should respond to change in currency (#654)

* Add step method to currency

* Change amount placeholder and step, when currency select change

* Lint

* Add test with auth

* Extract request to specific service

test/lib/money/currency_test.rb

@@ -46,4 +46,8 @@ class Money::CurrencyTest < ActiveSupport::TestCase
         assert_equal "12", value4.cents_str(2)
         assert_equal "123", value4.cents_str(3)
     end
+
+    test "step returns the smallest value of the currency" do
+      assert_equal 0.01, @currency.step
+    end
 end