Structures should handle UNSET_VALUE merging

lib/omniconfig/structure.rb

@@ -106,6 +106,12 @@ def value(raw)
     # The merge behavior for a structure is to do a deep merge, merging each
     # individual field within.
     def merge(old, new)
+      # We don't need to merge if one of the values is unset, since that is
+      # pretty straightforward.
+      return new if old == UNSET_VALUE
+      return old if new == UNSET_VALUE
+
+      # Merge member by member
       result = {}
       @members.each do |key, type|
         if old.has_key?(key) && !new.has_key?(key)

spec/omniconfig/structure_spec.rb

@@ -90,5 +90,13 @@
       result = instance.merge(old, new)
       result["foo"].should == 5
     end
+
+    it "should take the new value if the old one is UNSET" do
+      instance.merge(OmniConfig::UNSET_VALUE, 5).should == 5
+    end
+
+    it "should take the old value if the new one is UNSET" do
+      instance.merge(2, OmniConfig::UNSET_VALUE).should == 2
+    end
   end
 end