Change QSslCertificate::toText() to return a QString.

Change QSslCertificate::toText() to return a QString.

A couple of people reviewing the toText() method (which is new in 5.0)
have said that since the string returned is human readable it should
be a QString not a QByteArray. This change follows their advice.

Change-Id: Ibade9a24870805f7fbe2d299abeb9c6e964f0cf4
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>