Fix Money#allocate for enumerable of all zeros

spec/money/allocate_spec.cr

@@ -2,6 +2,12 @@ require "../spec_helper"
 
 describe Money::Allocate do
   describe "#allocate" do
+    context "with all zeros" do
+      it "allocates evenly" do
+        Money.us_dollar(100).allocate([0, 0]).map(&.cents).should eq [50, 50]
+      end
+    end
+
     it "takes no action when one gets all" do
       Money.us_dollar(5).allocate({1.0}).map(&.cents).should eq [5]
     end

src/money/money/allocate.cr

@@ -38,6 +38,10 @@ struct Money
       allocations = allocations_from_splits(splits)
       raise ArgumentError.new("Splits add to more then 100%") if allocations > 1.0
 
+      if splits.all?(&.zero?)
+        allocations = splits.size.to_f
+      end
+
       amounts, left_over = amounts_from_splits(allocations, splits)
       delta = left_over > 0 ? 1 : -1
       size = amounts.size