Added test case for an enum inside of an array

[tbartelmess]

Right now this PR just adds a test case to demonstrate an issue.

I am not 100%, but it looks like that enum types are just registered with a PG_FORMAT_TEXT. However when used inside of an array, it tries to re-register the type for both the PG_FORMAT_TEXT and PG_FORMAT_BINARY format, which then causes an issue an exception because it's already registeres for the text decoding.

await self.con.execute('''
    CREATE TYPE enum_t AS ENUM ('abc', 'def', 'ghi');
''')
result = await self.con.fetchrow('''SELECT ARRAY[$1::enum_t];''',
                                 'abc')

======================================================================
ERROR: test_enum_in_array (tests.test_codecs.TestCodecs)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/tbartelmess/Developer/asyncpg/asyncpg/_testbase.py", line 65, in wrapper
    self.loop.run_until_complete(__meth__(self, *args, **kwargs))
  File "/Users/tbartelmess/.pyenv/versions/3.6.0/lib/python3.6/asyncio/base_events.py", line 466, in run_until_complete
    return future.result()
  File "/Users/tbartelmess/Developer/asyncpg/tests/test_codecs.py", line 1062, in test_enum_in_array
    'abc')
  File "/Users/tbartelmess/Developer/asyncpg/asyncpg/connection.py", line 303, in fetchrow
    stmt = await self._get_statement(query, timeout)
  File "/Users/tbartelmess/Developer/asyncpg/asyncpg/connection.py", line 220, in _get_statement
    protocol.get_settings().register_data_types(types)
  File "asyncpg/protocol/settings.pyx", line 32, in asyncpg.protocol.protocol.ConnectionSettings.register_data_types (asyncpg/protocol/protocol.c:4675)
    cpdef inline register_data_types(self, types):
  File "asyncpg/protocol/settings.pyx", line 33, in asyncpg.protocol.protocol.ConnectionSettings.register_data_types (asyncpg/protocol/protocol.c:4598)
    self._data_codecs.add_types(types)
  File "asyncpg/protocol/codecs/base.pyx", line 470, in asyncpg.protocol.protocol.DataCodecConfig.add_types (asyncpg/protocol/protocol.c:17697)
    self.set_builtin_type_codec(oid, name, schema, 'scalar',
  File "asyncpg/protocol/codecs/base.pyx", line 494, in asyncpg.protocol.protocol.DataCodecConfig.set_builtin_type_codec (asyncpg/protocol/protocol.c:18314)
    raise ValueError('cannot override codec for type {}'.format(
ValueError: cannot override codec for type 16385

----------------------------------------------------------------------

[elprans]

Hi @tbartelmess. I just merged #89, which should fix this case. Feel free to rebase and resubmit the PR.

[elprans]

s/fetchrow/fetchval/

[1st1]

Merged by hand in bcb9de0.