Return actual secret strings for private keys when appear in same line

[arielkru]

Currently when private key secrets are detected, the secret value that is returned in the output refers only to the generic BEGIN PRIVATE KEY opening and not to the secret key itself.
Ideally this should be handled as a multiline secret, since its mostly includeing new lines within it and so the opening wrapper is inspected in a different scan_line iteration that the key itself.
For now, I added this change that returns the actual key, if the input is parsed in a way that escapes new line characters, like json.dumps(). So it still doesnt cover all of the use cases with the right secret value, but at least those that can be easily treated with the single line scanning method.

[jpdakran]

Hi. Please merge master to your branch to re-run CI.