[DSL] Added the missing aggregation

See: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-missing-aggregation.html

Related: #286, #287

Author: tmaier

lib/elasticsearch/dsl/search/aggregations/missing.rb

@@ -0,0 +1,35 @@
+module Elasticsearch
+  module DSL
+    module Search
+      module Aggregations
+
+        # A multi-bucket aggregation which returns the collection of terms and their document counts
+        #
+        # @example Passing the options as a Hash
+        #
+        #     aggregation :tags do
+        #       missing field: 'tags'
+        #     end
+        #
+        # @example Passing the options as a block
+        #
+        #     search do
+        #       aggregation :tags do
+        #         missing do
+        #           field 'tags'
+        #         end
+        #       end
+        #     end
+        #
+        # @see https://www.elastic.co/guide/en/elasticsearch/reference/master/search-aggregations-bucket-missing-aggregation.html
+        #
+        class Missing
+          include BaseAggregationComponent
+
+          option_method :field
+        end
+
+      end
+    end
+  end
+end

test/unit/aggregations/missing_test.rb

@@ -0,0 +1,39 @@
+require 'test_helper'
+
+module Elasticsearch
+  module Test
+    module Aggregations
+      class RangeTest < ::Test::Unit::TestCase
+        include Elasticsearch::DSL::Search::Aggregations
+
+        context "Missing aggregation" do
+          subject { Missing.new }
+
+          should "be converted to a Hash" do
+            assert_equal({ missing: {} }, subject.to_hash)
+          end
+
+          should "take a Hash" do
+            subject = Missing.new( { field: 'test' } )
+            assert_equal({ missing: { field: "test" } }, subject.to_hash)
+          end
+
+          should "have option methods" do
+            subject.field 'foo'
+
+            assert_equal %w[ field ], subject.to_hash[:missing].keys.map(&:to_s).sort
+            assert_equal 'foo', subject.to_hash[:missing][:field]
+          end
+
+          should "take a block" do
+            subject = Missing.new do
+              field 'bar'
+            end
+
+            assert_equal({missing: { field: 'bar' } }, subject.to_hash)
+          end
+        end
+      end
+    end
+  end
+end