Revert "Drop support for initialization with another money object."

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    shopify-money (2.0.0)
+    shopify-money (2.1.0)
 
 GEM
   remote: https://rubygems.org/

lib/money/helpers.rb

@@ -16,6 +16,8 @@ module Helpers
     def value_to_decimal(num)
       value =
         case num
+        when Money
+          num.value
         when BigDecimal
           num
         when nil, 0, ''

spec/helpers_spec.rb

@@ -7,8 +7,8 @@
     let (:amount) { BigDecimal('1.23') }
     let (:money) { Money.new(amount) }
 
-    it 'raises when provided with a money object' do
-      expect { subject.value_to_decimal(money) }.to raise_error(ArgumentError)
+    it 'returns the value of a money object' do
+      expect(subject.value_to_decimal(money)).to eq(amount)
     end
 
     it 'returns itself if it is already a big decimal' do