Ensure that plucky is not destructive with array of id inputs.

Fixes #15

lib/plucky/criteria_hash.rb

@@ -122,7 +122,7 @@ def normalized_value(parent_key, key, value)
         case value
           when Array, Set
             if object_id?(parent_key)
-              value.map! { |v| Plucky.to_object_id(v) }
+              value.map { |v| Plucky.to_object_id(v) }
             elsif NestingOperators.include?(key)
               value.map  { |v| CriteriaHash.new(v, options).to_hash }
             elsif parent_key == key
@@ -145,4 +145,4 @@ def normalized_value(parent_key, key, value)
         end
       end
   end
-end
+end

test/plucky/test_criteria_hash.rb

@@ -260,12 +260,17 @@ class CriteriaHashTest < Test::Unit::TestCase
       setup do
         @id1      = BSON::ObjectId.new
         @id2      = BSON::ObjectId.new
-        @criteria = CriteriaHash.new({:_id => {'$in' => [@id1.to_s, @id2.to_s]}}, :object_ids => [:_id])
+        @ids      = [@id1.to_s, @id2.to_s]
+        @criteria = CriteriaHash.new({:_id => {'$in' => @ids}}, :object_ids => [:_id])
       end
 
       should "convert strings to object ids" do
         @criteria[:_id].should == {'$in' => [@id1, @id2]}
       end
+
+      should "not modify original array of string ids" do
+        @ids.should == [@id1.to_s, @id2.to_s]
+      end
     end
 
     context "#merge" do
@@ -347,4 +352,4 @@ class CriteriaHashTest < Test::Unit::TestCase
       end
     end
   end
-end
+end