Return X509Certificate[] from SSLSession.getPeerCertificates()

[cconlon]

This PR adjusts SSLSession.getPeerCertificates() to return an array of typeX509Certificate rather than Certificate. Previously wolfJSSE created individual array elements as java.security.cert.X509Certificate objects, but returned them as a java.security.cert.Certificate[] to conform to the Javadoc specification for SSLSession.getPeerCertificates():

Certificate[] getPeerCertificates() throws SSLPeerUnverifiedException

Some callers of this API (for example Postgres) tries to cast this return back to an array of X509Certificate[], such as:

SSLSession session;
X509Certificate[] peerCerts;
...
peerCerts = (X509Certificate[]) session.getPeerCertificates();

This cast will throw an Exception similar to:

java.lang.ClassCastException: class [Ljava.security.cert.Certificate; cannot be cast to class [Ljava.security.cert.X509Certificate;

This PR adjusts WolfSSLImplementSSLSession.getPeerCertificates() to return a X509Certificate[] from the body of the method, to match SunJSSE behavior. Since X509Certificate is a subclass of Certificate, this should not break calling applications. Method signature is left intact as returning a Certificate[], so we still conform to SSLSession definition in general.