[API] Fix the path for indices exists_type? method & update docs

Closes #461

Author: Dawn Richardson

elasticsearch-api/lib/elasticsearch/api/actions/indices/exists_type.rb

@@ -22,7 +22,7 @@ module Actions
         # @option arguments [Boolean] :local Return local information, do not retrieve the state from master node
         #                                    (default: false)
         #
-        # @see http://www.elasticsearch.org/guide/reference/api/admin-indices-types-exists/
+        # @see https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-types-exists.html
         #
         def exists_type(arguments={})
           raise ArgumentError, "Required argument 'index' missing" unless arguments[:index]
@@ -36,7 +36,7 @@ def exists_type(arguments={})
           ]
 
           method = HTTP_HEAD
-          path   = Utils.__pathify Utils.__listify(arguments[:index]), Utils.__escape(arguments[:type])
+          path   = Utils.__pathify Utils.__listify(arguments[:index]), '_mapping', Utils.__escape(arguments[:type])
 
           params = Utils.__validate_and_extract_params arguments, valid_params
           body = nil

elasticsearch-api/test/unit/indices/exists_type_test.rb

@@ -10,7 +10,7 @@ class IndicesExistsTypeTest < ::Test::Unit::TestCase
         should "perform correct request" do
           subject.expects(:perform_request).with do |method, url, params, body|
             assert_equal 'HEAD', method
-            assert_equal 'foo/bar', url
+            assert_equal 'foo/_mapping/bar', url
             assert_equal Hash.new, params
             assert_nil   body
             true
@@ -21,7 +21,7 @@ class IndicesExistsTypeTest < ::Test::Unit::TestCase
 
         should "perform request against multiple indices" do
           subject.expects(:perform_request).with do |method, url, params, body|
-            assert_equal 'foo,bar/bam', url
+            assert_equal 'foo,bar/_mapping/bam', url
             true
           end.returns(FakeResponse.new)
 
@@ -30,7 +30,7 @@ class IndicesExistsTypeTest < ::Test::Unit::TestCase
 
         should "URL-escape the parts" do
           subject.expects(:perform_request).with do |method, url, params, body|
-            assert_equal 'foo%5Ebar/bar%2Fbam', url
+            assert_equal 'foo%5Ebar/_mapping/bar%2Fbam', url
             true
           end.returns(FakeResponse.new)