Allow cert parameter as array in config

[wimme002]

• [ ] Bug fix
• [x] New feature
• [ ] BC breaks
• [ ] Deprecations
• [x] Tests pass

Guzzle allows both an array or a string as cert parameter https://github.com/guzzle/guzzle/blob/master/src/Handler/CurlFactory.php#L407

License: MIT

[florianpreusner]

@wimme002, thanks a lot for your contribution! 🎉