Fix existing methods override

Fix issue elastic#924

more:
elastic#936

Author: George Papanikolaou

elasticsearch-model/lib/elasticsearch/model.rb

@@ -112,7 +112,7 @@ def self.included(base)
         # Delegate common methods to the `__elasticsearch__` ClassMethodsProxy, unless they are defined already
         class << self
           METHODS.each do |method|
-            delegate method, to: :__elasticsearch__ unless self.respond_to?(method)
+            delegate method, to: :__elasticsearch__ unless self.public_instance_methods.include?(method)
           end
         end
       end

elasticsearch-model/spec/elasticsearch/model/module_spec.rb

@@ -48,6 +48,7 @@ def self.search(query, options={})
       end
 
       DummyIncludingModel.__send__ :include, Elasticsearch::Model
+      DummyIncludingModelWithSearchMethodDefined.__send__ :include, Elasticsearch::Model
     end
 
     after(:all) do