sslmode should default to "prefer" not "disable" #654

• asyncpg version: asyncpg==0.21.0
• PostgreSQL version: 12.5
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: Heroku, yes
• Python version: 3.8.6
• Platform: macos and ubuntu on heroku
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: NA
• Can the issue be reproduced under both asyncio and
  uvloop?: yes (I guess)

The docstring for connect says:

(the associated link to the docs is broken, but I've fixed that in #653, should link to here)

But this is not correct, currently asyncpg defaults to the equivalent of sslmode=disable, e.g. it doesn't try to use SSL for the connection, while the linked postgresql.org docs says, regarding sslmode:

prefer (default) first try an SSL connection; if that fails, try a non-SSL connection

e.g. prefer is the default.

asyncpg too should default to prefer.

This caused a connection error when upgrading a heroku database from hobby tier to standard - the latter requires SSL.

From reading the source, it looks like the best work around for now will be to set the environment variable PGSSLMODE=prefer: