in_http: Add a test case for the keep_time_key flag.

This patch adds a test case (test_multi_json_with_keep_time_key) to
test_in_http.rb.

I've confirmed it passes without problems in my testing machine:

    test_multi_json_with_keep_time_key:            .: (0.606005)

Signed-off-by: Fujimoto Seiji <fujimoto@clear-code.com>

test/plugin/test_in_http.rb

@@ -347,6 +347,34 @@ def test_json_with_add_http_headers
     assert include_http_header?(d.events[1][2])
   end
 
+  def test_multi_json_with_keep_time_key
+    d = create_driver(CONFIG + %[
+        <parse>
+          @type json
+          keep_time_key true
+        </parse>
+    ])
+    time1 = event_time("2011-01-02 13:14:15 UTC")
+    time2 = event_time("2012-01-02 13:14:15 UTC")
+    records = [{"time"=>time1.to_i},{"time"=>time2.to_i}]
+    tag = "tag1"
+    res_codes = []
+
+    d.run(expect_records: 2, timeout: 5) do
+      res = post("/#{tag}", records.to_json, {"Content-Type"=>"application/octet-stream"})
+      res_codes << res.code
+    end
+    assert_equal ["200"], res_codes
+
+    assert_equal "tag1", d.events[0][0]
+    assert_equal_event_time time1, d.events[0][1]
+    assert_equal d.events[0][2], records[0]
+
+    assert_equal "tag1", d.events[1][0]
+    assert_equal_event_time time2, d.events[1][1]
+    assert_equal d.events[1][2], records[1]
+  end
+
   def test_application_json
     d = create_driver
     time = event_time("2011-01-02 13:14:15 UTC")