Selecting schema not worked with pgbouncer

Describe the bug
Get error from asyncpg when set custom schema in connection credentials:
asyncpg.exceptions.ProtocolViolationError: unsupported startup parameter: search_path

To Reproduce
CREATE SCHEMA IF NOT EXISTS notpublic

await Tortoise.init(
    {
        "connections": {
            __name__: {
                "engine": "tortoise.backends.asyncpg",
                "credentials": {
                    **credentials,
                    "schema": "notpublic"
                }
            }
        },
        "apps": {"models": {"models": [__name__], "default_connection": __name__}},
    },
)

Expected behavior
Successfully init

Additional context
Support schema added in https://github.com/tortoise/tortoise-orm/pull/172 and changed in https://github.com/tortoise/tortoise-orm/pull/229

Start by reproducing the Tortoise.init configuration with asyncpg, the custom schema credential, and PgBouncer, then trace how the schema becomes the PostgreSQL startup parameter. Confirm the fix by successfully initializing through PgBouncer and adding regression coverage for this configuration.