RSAKey.toRSAPrivateKey: Consider returning PrivateKey field when available

[Vladimir Dzhuvinov]

Instead of recreating the key object from private key material.

[Vladimir Dzhuvinov]

E.g. to conserve additional PSS data:

SunRsaSign RSASSA-PSS private CRT key, 2048 bits
  params: MD: SHA-256
MGF: java.security.spec.MGF1ParameterSpec@612679d6
SaltLength: 32
TrailerField: 1

vs

SunRsaSign RSA private CRT key, 2048 bits
  params: null

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