[DSL] Fixed the documentation and naming for the missing aggregation in 821576d

Closes #286
Closes #287

Author: karmi

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

@@ -3,18 +3,19 @@ module DSL
     module Search
       module Aggregations
 
-        # A multi-bucket aggregation which returns the collection of terms and their document counts
+        # A single bucket aggregation that creates a bucket of all documents
+        # which are missing a value for the field
         #
         # @example Passing the options as a Hash
         #
-        #     aggregation :tags do
+        #     aggregation :articles_without_tags do
         #       missing field: 'tags'
         #     end
         #
         # @example Passing the options as a block
         #
         #     search do
-        #       aggregation :tags do
+        #       aggregation :articles_without_tags do
         #         missing do
         #           field 'tags'
         #         end

elasticsearch-dsl/test/unit/aggregations/missing_test.rb

@@ -3,7 +3,7 @@
 module Elasticsearch
   module Test
     module Aggregations
-      class RangeTest < ::Test::Unit::TestCase
+      class MissingTest < ::Test::Unit::TestCase
         include Elasticsearch::DSL::Search::Aggregations
 
         context "Missing aggregation" do
@@ -14,8 +14,8 @@ class RangeTest < ::Test::Unit::TestCase
           end
 
           should "take a Hash" do
-            subject = Missing.new( { field: 'test' } )
-            assert_equal({ missing: { field: "test" } }, subject.to_hash)
+            subject = Missing.new( { field: 'foo' } )
+            assert_equal({ missing: { field: "foo" } }, subject.to_hash)
           end
 
           should "have option methods" do