test_parser_v10: fix unittest breakage

Commit 366da0 break unittest.
The following error occurs.

$ ./run_tests.sh
(...)

======================================================================
ERROR: test_parser (ryu.tests.unit.ofproto.test_parser_v10.TestOFPVendorStatsReply)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/hideki/ryu/ryu/tests/unit/ofproto/test_parser_v10.py", line 4962, in test_parser
    eq_(specific_data, body.specific_data)
AttributeError: 'str' object has no attribute 'specific_data'

----------------------------------------------------------------------
Ran 1249 tests in 0.991s

FAILED (errors=1)

Signed-off-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>