Allow adapters to be passed as arguments of cursor's execute()

[amigrave]

psycopg2 allows to pass Adapter instances as arguments of cursor's
execute(). This patch will make psycopg2cffi behave the same way.

Here's a test case that can be run against psycopg2 and psycopg2cffi

import psycopg2  # assuming psycopg2cffi-compat
from psycopg2.extensions import AsIs, Boolean

conn = psycopg2.connect(database='postgres')
cr = conn.cursor()
cr.execute("SELECT %s, %s", (AsIs("version()"), Boolean(True)))
print(cr.fetchone())

Basically psycopg2cffi does not allow users to pass an adapter in the argument list of execute() because the values will be converted/adapted no matter what.

This commit will check if the value is already an Adapter instance before trying to adapt it.

[lopuhin]

Thanks for PR @amigrave , the changes look good. Could you please add a test as well?

[lopuhin]

Thanks @amigrave and @thedrow