Problem using PuTTYKeyFile

Hi

I recently noticed the support for PuTTY Key File handling in SSHJ and am now using it.

However I have a further requirement to look at Putty Key file handling outside of SSHJ e.g. extract public key.

For this I have been looking at using the PuTTYKeyFile class directly, but I hit a few problems.

So is PuTTYKeyFile intended for API usage? I appreciate this is possibly engineering to be used under the covers within SSHJ.

If so then can I make a few observations that would improve it?

My main issue is when check a file (.ppk) to determine if it is encrypted.

I see that init() and isEncrypted() methods are public so should I be able to do this?
```
PuTTYKeyFile pkf = new PuTTYKeyFile();
pkf.init(new File("C:/myEncryptedPuttyFile.ppk"));
System.out.println("File is encrypted["+pkf.isEncrypted()+"]");
```

However with this I always get false.

The reason appears to be that isEncrypted() (which is public) inspects "headers" map, but used this way this map is empty. Digging a bit deeper I can see that "headers" is populated in "parseKeyPair()", but this is protected and therefore can't be directly called.

Now, as I say, it's possible that you never intended this class to be used externally from SSHJ, so should isEncrytped() should be "protected" scope rather than "public"?

But I think with a few tweaks this could be a very useful utility - if you agree then I could possibly take a deeper look and provide a pull request.

For my requirement I need to check the file is encrypted, before prompting user then supplying calling "init(File location, PasswordFinder pwdf)" to specify the password later.