[CurlFactory] Prevent undefined offset when using array for ssl_key options

[andrewnclark]

Hopefully resolves #2339 where the ssl_key options are provided using the array format, but without a password as the second item in the array.

Still waiting conformation that this was the cause of the warnings, but this will avoid them going forward.

As always, feedback is appreciated.

[andrewnclark]

Has since been confirmed that the cause was options defined as such:

$options = [
    'ssl_key' => ['/some/file/path']
]

This PR will prevent further warnings but the alternative is to validate properly that the options are both provided in the array or use a single string, thus making the above invalid and prompting a new release and update instructions.

[sagikazarmark]

For maximum compatibility, I think it should be possible to use the array format without a key (even if it's not intended to be supported). We can deprecate that behavior though, and remove it in the next major release.

@andrewnclark are you up for making that change?

[sagikazarmark]

Also, can you please rebase your branch from master?

[Nyholm]

I fixed the bug discovered by the test and I rebased the PR.

[sagikazarmark]

Thanks!

[Nyholm]

Thank you for the reviews and for the fix