AsyncHTTPTestCase's fetch swallows httpclient exception

Python: 3.5.1
Tornado: 4.3
Ubuntu 12.04

Case:

import unittest
from tornado import testing
from tornado import web
from tornado import gen

class Handler(web.RequestHandler):

    @gen.coroutine
    def post(self):
        self.write('spam')


class HandlerTest(testing.AsyncHTTPTestCase):
    def get_app(self):
        return web.Application([('/', Handler)])

    def test_post(self):
        response = self.fetch('/', method='POST')
        self.assertEqual(b'spam', response.body)

if __name__ == '__main__':
    unittest.main()

Produces:

F
======================================================================
FAIL: test_post (__main__.HandlerTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/t35/lib/python3.5/site-packages/tornado/testing.py", line 132, in __call__
    result = self.orig_method(*args, **kwargs)
  File "test.py", line 20, in test_post
    self.assertEqual(b'spam', response.body)
AssertionError: b'spam' != None

----------------------------------------------------------------------
Ran 1 test in 0.005s

FAILED (failures=1)

instead of "meaningful" error.

If changed to @gen_test, it produce proper error:

import unittest
from tornado import testing
from tornado import web
from tornado import gen
from tornado.testing import gen_test

class Handler(web.RequestHandler):

    @gen.coroutine
    def post(self):
        self.write('spam')


class HandlerTest(testing.AsyncHTTPTestCase):
    def get_app(self):
        return web.Application([('/', Handler)])

    @gen_test
    def test_post(self):
        response = self.http_client.fetch(self.get_url('/'), method='POST')
        self.assertEqual(b'spam', response.body)

if __name__ == '__main__':
    unittest.main()

and the error, that is ok:

E
======================================================================
ERROR: test_post (__main__.HandlerTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/t35/lib/python3.5/site-packages/tornado/testing.py", line 132, in __call__
    result = self.orig_method(*args, **kwargs)
  File "/tmp/t35/lib/python3.5/site-packages/tornado/testing.py", line 525, in post_coroutine
    timeout=timeout)
  File "/tmp/t35/lib/python3.5/site-packages/tornado/ioloop.py", line 453, in run_sync
    return future_cell[0].result()
  File "/tmp/t35/lib/python3.5/site-packages/tornado/concurrent.py", line 232, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 3, in raise_exc_info
  File "/tmp/t35/lib/python3.5/site-packages/tornado/gen.py", line 267, in wrapper
    result = func(*args, **kwargs)
  File "/tmp/t35/lib/python3.5/types.py", line 243, in wrapped
    coro = func(*args, **kwargs)
  File "/tmp/t35/lib/python3.5/site-packages/tornado/testing.py", line 508, in pre_coroutine
    result = f(self, *args, **kwargs)
  File "test.py", line 22, in test_post
    self.assertEqual(b'spam', response.body)
AttributeError: 'Future' object has no attribute 'body'

----------------------------------------------------------------------
Ran 1 test in 0.006s

FAILED (errors=1)
ERROR:tornado.application:Future <tornado.concurrent.Future object at 0x7ffb966965c0> exception was never retrieved: Traceback (most recent call last):
  File "/tmp/t35/lib/python3.5/site-packages/tornado/stack_context.py", line 314, in wrapped
    ret = fn(*args, **kwargs)
  File "/tmp/t35/lib/python3.5/site-packages/tornado/gen.py", line 264, in <lambda>
    future, lambda future: callback(future.result()))
  File "/tmp/t35/lib/python3.5/site-packages/tornado/simple_httpclient.py", line 353, in _on_connect
    ('not ' if body_expected else '', self.request.method))
ValueError: Body must not be None for method POST (unless allow_nonstandard_methods is true)

Ref. http://stackoverflow.com/questions/36013805/can-only-perform-get-requests-in-tornado-test/36015750#36015750

Reproduce the shown test_post case, then inspect AsyncHTTPTestCase.fetch in tornado/testing.py and the exception path in tornado/simple_httpclient.py. Add regression coverage for this failure and verify that fetch surfaces the underlying HTTP-client error instead of returning a response with body set to None.