"Cannot insert multiple commands into prepared statement" issue while executing query with params

まだ誰も着手していません。

(python 3.5.1, asyncpg 0.5.4)

Consider the next code

```
import asyncpg
import asyncio

async def run():
con = await asyncpg.connect(user="coldmind", database="test")
await con.execute('SELECT ($1); SELECT 2;', 1)
asyncio.get_event_loop().run_until_complete(run())
```

The executing fails with `asyncpg.exceptions.PostgresSyntaxError: cannot insert multiple commands into a prepared statement` error, while the another example (without passing params to the query) works well with multiple commands:

```
import asyncpg
import asyncio

async def run():
con = await asyncpg.connect(user="coldmind", database="test")
await con.execute('SELECT 1; SELECT 2;')
asyncio.get_event_loop().run_until_complete(run())
```

Why is the first example using a prepared statement instead of executing it?

Start with the con.execute entry point and run the two Python examples from the issue to compare parameterized and non-parameterized queries. Trace how parameters affect execution, then establish the expected behavior for multiple commands and document or test the resulting decision.

索引モデルが issue の本文から書いたものです。