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

(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.

Written by the indexing model from the issue text.