Include 'Content-Type' header in transport_options

Starting with Elasticsearch 5.3, rest requests without an explicit
Content-Type are deprecated and it'll log a warning for every request
that is sent without a Content-Type header.

I addressed this in a similar manner to how `elasticsearch-ruby` did,
see [1], [2].

[1]: elastic/elasticsearch-ruby#400
[2]: elastic/elasticsearch-ruby@76f8679

lib/fluent/plugin/out_elasticsearch.rb

@@ -150,6 +150,7 @@ def client
                                                                             resurrect_after: @resurrect_after,
                                                                             retry_on_failure: 5,
                                                                             transport_options: {
+                                                                              headers: { 'Content-Type' => 'application/json' },
                                                                               request: { timeout: @request_timeout },
                                                                               ssl: { verify: @ssl_verify, ca_file: @ca_file }
                                                                             }

lib/fluent/plugin/out_elasticsearch_dynamic.rb

@@ -48,6 +48,7 @@ def client(host)
                                                                             resurrect_after: @resurrect_after,
                                                                             retry_on_failure: 5,
                                                                             transport_options: {
+                                                                              headers: { 'Content-Type' => 'application/json' },
                                                                               request: { timeout: @request_timeout },
                                                                               ssl: { verify: @ssl_verify, ca_file: @ca_file }
                                                                             }

test/plugin/test_out_elasticsearch.rb

@@ -326,6 +326,16 @@ def test_single_host_params_and_defaults
     assert_equal nil, host1[:path]
   end
 
+  def test_content_type_header
+    stub_request(:head, "http://localhost:9200/").
+      to_return(:status => 200, :body => "", :headers => {})
+    elastic_request = stub_request(:post, "http://localhost:9200/_bulk").
+      with(headers: { "Content-Type" => "application/json" })
+    driver.emit(sample_record)
+    driver.run
+    assert_requested(elastic_request)
+  end
+
   def test_writes_to_default_index
     stub_elastic_ping
     stub_elastic

test/plugin/test_out_elasticsearch_dynamic.rb

@@ -154,6 +154,16 @@ def test_single_host_params_and_defaults
     assert_equal nil, host1[:path]
   end
 
+  def test_content_type_header
+    stub_request(:head, "http://localhost:9200/").
+      to_return(:status => 200, :body => "", :headers => {})
+    elastic_request = stub_request(:post, "http://localhost:9200/_bulk").
+      with(headers: { "Content-Type" => "application/json" })
+    driver.emit(sample_record)
+    driver.run
+    assert_requested(elastic_request)
+  end
+
   def test_writes_to_default_index
     stub_elastic_ping
     stub_elastic