Expected private/secret keys in the source (TIKA-3205)

[Nick Burch]

Hey All

Just a quick heads-up that for TIKA-3205 I generated a few new small 
private keys (RSA, DSA, EC) and added them to the parser test documents 
folder, for unit testing the new mime magics for keys and certificates. 
They're not protecting or using anything.

One automated security scanning tool has already emailed me to warn that I 
committed secrets (GitGuardian), and I think there's a chance others might 
do too...

So before anyone else gets a notification and worries, I felt it best to 
give everyone a heads-up that yes, there are private key files in the Tika 
source tree, and yes, they are supposed to be there!

Cheers
Nick