Added headers() and setHeaders() methods to QNetworkRequest.
[ChangeLog][QtNetwork][QNetworkRequest] Added headers() and setHeaders()
methods to QNetworkRequest, which provide an interface to work with
QHttpHeaders.
Task-number: QTBUG-107751
Change-Id: I2e1dc7cb2efab5903eb7ff23b75d01aefe13273d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Lena Biliaieva