[MODEL] Fixed #dup behaviour for Elasticsearch::Model

Previously, calling #dup for an Elasticsearch::Model instance would
retain the original __elasticsearch__ reference. Given the following
example:

    user = User.create!(name: "Will")
    other = user.dup
    other.update!(name: "Bill")

You'd end up with two references to "Will" in Elasticsearch, and none
for "Bill," because the duplicate instance proxied to the original
instance's attributes.

With this fix, each duplicate gets its own proxy, so attributes are
saved correctly.

Closes elastic#517

Author: jferris

elasticsearch-model/lib/elasticsearch/model/proxy.rb

@@ -66,6 +66,15 @@ def __elasticsearch__ &block
         end
       end
 
+      # @overload dup
+      #
+      # Returns a copy of this object. Resets the __elasticsearch__ proxy so
+      # the duplicate will build its own proxy.
+      def initialize_dup(_)
+        @__elasticsearch__ = nil
+        super
+      end
+
       # Common module for the proxy classes
       #
       module Base

elasticsearch-model/test/unit/proxy_test.rb

@@ -69,6 +69,17 @@ def changes
       assert_equal 'insta barr', DummyProxyModel.new.__elasticsearch__.bar
     end
 
+    should "reset the proxy target for duplicates" do
+      model = DummyProxyModel.new
+      model_target = model.__elasticsearch__.target
+      duplicate = model.dup
+      duplicate_target = duplicate.__elasticsearch__.target
+
+      assert_not_equal model, duplicate
+      assert_equal model, model_target
+      assert_equal duplicate, duplicate_target
+    end
+
     should "return the proxy class from instance proxy" do
       assert_equal Elasticsearch::Model::Proxy::ClassMethodsProxy, DummyProxyModel.new.__elasticsearch__.class.class
     end