Ensure we return nils for unset keys in Dash

In 0bd18ee, we stopped setting any `nil` values on the Dash in order to
prevent co-dependent properties from throwing errors. This is a problem
because it means that when you explicitly set `nil` as a value for
a property, it's not set on the underlying hash and you will not get it
back if you call `#to_h`.

By explicitly setting a nil value on the result of `#to_h` for unset
keys, we can avoid this issue and return what the called expects.

lib/hashie/dash.rb

@@ -183,6 +183,15 @@ def update_attributes!(attributes)
       assert_required_attributes_set!
     end
 
+    def to_h
+      result = super
+      self.class.properties.each do |property|
+        result[property] ||= nil
+      end
+      yield result if block_given?
+      result
+    end
+
     private
 
     def initialize_attributes(attributes)

spec/hashie/dash_spec.rb

@@ -446,6 +446,15 @@ class KeepingMash < Hashie::Mash
       )
     end
   end
+
+  describe '#to_hash' do
+    it 'does not write out keys with nil values' do
+      expect(DashTest.new(first_name: 'Bob', email: nil).to_hash).to eq(
+        first_name: 'Bob',
+        count: 0
+      )
+    end
+  end
 end
 
 describe Hashie::Dash, 'inheritance' do