code: def testPlainText_appendsUtf8ToContentType(self):
      q = wrappers.Request(wtest.EnvironBuilder().get_environ())
      r = http_util.Respond(q, "hello", "text/plain")
      h = r.headers
      self.assertEqual(
          h.get("Content-Type"), "text/plain; charset=utf-8"
      )
  
imports: import gzip
  import io
  import struct
  from unittest import mock
  from werkzeug import test
  from werkzeug import wrappers
  from tensorboard import test
  from tensorboard.backend import http_util
label_desc: Use image node_id 3 for calling the RespondTest obj's underlying member method code with example usage: obj.testPlainText_appendsUtf8ToContentType() without return types