set_type_codec() doesn't work with copy_records_to_table()

• asyncpg version: 0.23.0
• PostgreSQL version: 9.5.21
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: No and this is on a local PSQL install.
• Python version: 3.6.13
• Platform: Ubuntu 16.04.6 LTS
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: n/a
• Can the issue be reproduced under both asyncio and
  uvloop?: Not tried

The following:

    await conn.set_type_codec(
        "numeric",
        encoder=str,
        decoder=float,
        schema="pg_catalog",
    )

Doesn't work when used with copy_records_to_table()

 ...
 File "asyncpg/protocol/protocol.pyx", line 504, in copy_in
  File "asyncpg/protocol/protocol.pyx", line 437, in asyncpg.protocol.protocol.BaseProtocol.copy_in
asyncpg.exceptions._base.InternalClientError: no binary format encoder for type numeric (OID 1700)

This is despite solving issue https://github.com/MagicStack/asyncpg/issues/157 which added a binary format codec for numeric types - https://github.com/MagicStack/asyncpg/commit/0f8483517ebde66f3c7cc858fa6ad227c6e3fd64

Can we get the ability to provide a custom decoder without having to also provide an encoder then internally just fall back to the default encoder? I believe it should solve the issue. E.g.

    await conn.set_type_codec(
        "numeric",
        decoder=float,
        schema="pg_catalog",
    )

or

    await conn.set_type_codec(
        "numeric",
        encoder=None,
        decoder=float,
        schema="pg_catalog",
    )

使用 issue 中的数值示例，通过 set_type_codec() 和 copy_records_to_table() 重现该失败。先从 asyncpg/protocol/protocol.pyx 中的 copy_in 和 BaseProtocol.copy_in 开始，然后跟踪自定义 codec 的选择过程。当可以使用自定义解码器，同时 copy_records_to_table() 仍能获取 numeric 的编码器时，即视为完成。