Trying to connect to RDS with SSL and getting stumped

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## What did you do?

Followed instructions here on how to get a trusted certificate from AWS for RDS.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_PostgreSQL.html#PostgreSQL.Concepts.General.SSL

Downloaded this cert...
https://s3.amazonaws.com/rds-downloads/rds-combined-ca-bundle.pem

Imported on macos to keychain and made it trusted.

## What did you expect to happen?

Expected to be able connect.

## What actually happened?

When I select the .pem file within client SSL option in Postico connection settings it fails to connect indicating it can't find a start line.

I don't think it's a bug, I think I'm missing a step or something...

## What software versions are you using?

Postico version: 1.5.4
(select "About Postico" from the Postico menu)

macOS version: 10.13.6
(selecting "About this Mac" from the Apple menu)

PostgreSQL version: AWS RDS (not sure of the version as I don't want to connect w/oSSL from hotel)...
(execute the command "SELECT VERSION();" to find out)

Start with the AWS RDS PostgreSQL SSL instructions and the rds-combined-ca-bundle.pem file, then reproduce the failure using Postico 1.5.4 on macOS with the client SSL setting. Done means determining whether the certificate format or the SSL setting is unsupported and documenting or correcting the connection path.

Written by the indexing model from the issue text.