removed unused kwarg "delimiter"

It was originally for JSONL (json string per lines).
But currently it isn't needed because Yajl parser can process it in #read_message method.
I added a test case to confirm it.

lib/fluent/plugin/in_forward.rb

@@ -551,24 +551,8 @@ def on_connect
       end
 
       # API to register callback for data arrival
-      def on_data(delimiter: nil, &callback)
-        if delimiter.nil?
-          @on_read_callback = callback
-          return
-        end
-
-        # buffering and splitting
-        @buffer = "".force_encoding("ASCII-8BIT")
-        @on_read_callback = ->(data) {
-          @buffer << data
-          pos = 0
-          while i = @buffer.index(delimiter, pos)
-            msg = @buffer[pos...i]
-            callback.call(msg)
-            pos = i + delimiter.length
-          end
-          @buffer.slice!(0, pos) if pos > 0
-        }
+      def on_data(&callback)
+        @on_read_callback = callback
       end
 
       def on_read(data)

test/plugin/test_in_forward.rb

@@ -187,6 +187,27 @@ def test_json
 
       assert_equal(records, d.emits.sort_by {|a| a[1] })
     end
+
+    def test_json_with_newline
+      d = create_driver
+
+      time = Time.parse("2011-01-02 13:14:15 UTC").to_i
+
+      records = [
+        ["tag1", time, {"a"=>1}],
+        ["tag2", time, {"a"=>2}],
+      ]
+
+      d.expected_emits_length = records.length
+      d.run_timeout = 2
+      d.run do
+        records.each {|tag, _time, record|
+          send_data [tag, _time, record].to_json + "\n"
+        }
+      end
+
+      assert_equal(records, d.emits.sort_by {|a| a[1] })
+    end
   end
 
   class Forward < self