This library doesnt support private keys with headers

[shaybenh7]

Hey,
when I'm trying to create my JWT with my private key that contains the headers:
-----BEGIN RSA PRIVATE KEY-----
and
-----END RSA PRIVATE KEY-----
it gets an exception:
System.IO.IOException: END RSA PRIVATE KEY was not found.
at Org.BouncyCastle.Utilities.IO.Pem.PemReader.ReadPemObject()
at Org.BouncyCastle.OpenSsl.PemReader.ReadObject()

when removing it, it works.
would you be able to fix it?

[shaybenh7]

please review
#86