[MODEL] Fixed the incorrect protection for existing model methods (such as search)

Due to the different scope in the `class << self`, the `respond_to?` condition always returned false,
even when the model had in fact defined the method in question (such as `search`).

Instead of using `respond_to?`, `self.public_instance_methods.include?` has been used.

The concrete example is eg. [Ransack's](https://github.com/activerecord-hackery/ransack) `search` method.

Closes elastic#96

Author: karmi

elasticsearch-model/lib/elasticsearch/model.rb

@@ -62,6 +62,7 @@ module Elasticsearch
   #     # ...
   #
   module Model
+    METHODS = [:search, :mapping, :mappings, :settings, :index_name, :document_type, :import]
 
     # Adds the `Elasticsearch::Model` functionality to the including class.
     #
@@ -107,13 +108,9 @@ def as_indexed_json(options={})
         # Delegate important methods to the `__elasticsearch__` proxy, unless they are defined already
         #
         class << self
-          delegate :search,        to: :__elasticsearch__ unless respond_to?(:search)
-          delegate :mapping,       to: :__elasticsearch__ unless respond_to?(:mapping)
-          delegate :mappings,      to: :__elasticsearch__ unless respond_to?(:mappings)
-          delegate :settings,      to: :__elasticsearch__ unless respond_to?(:settings)
-          delegate :index_name,    to: :__elasticsearch__ unless respond_to?(:index_name)
-          delegate :document_type, to: :__elasticsearch__ unless respond_to?(:document_type)
-          delegate :import,        to: :__elasticsearch__ unless respond_to?(:import)
+          METHODS.each do |method|
+            delegate method, to: :__elasticsearch__ unless self.public_instance_methods.include?(method)
+          end
         end
 
         # Mix the importing module into the proxy

elasticsearch-model/test/unit/module_test.rb

@@ -22,6 +22,11 @@ class Elasticsearch::Model::ModuleTest < Test::Unit::TestCase
 
     context "when included in module/class, " do
       class ::DummyIncludingModel; end
+      class ::DummyIncludingModelWithSearchMethodDefined
+        def self.search(query, options={})
+          "SEARCH"
+        end
+      end
 
       should "include and set up the proxy" do
         DummyIncludingModel.__send__ :include, Elasticsearch::Model
@@ -40,6 +45,12 @@ class ::DummyIncludingModel; end
         assert_respond_to DummyIncludingModel, :document_type
         assert_respond_to DummyIncludingModel, :import
       end
+
+      should "not override existing method" do
+        DummyIncludingModelWithSearchMethodDefined.__send__ :include, Elasticsearch::Model
+
+        assert_equal 'SEARCH', DummyIncludingModelWithSearchMethodDefined.search('foo')
+      end
     end
 
   end