cf. RFC7230 section 6.7, an Upgrade header in a normal response merely informs the client that the server supports upgrading to other protocols, and the client can ask for such an upgrade in a later request. The server requiring an upgrade is via the 426 Upgrade Required response code, not the mere presence of the Upgrade response header.

(closes issue #5573)

Signed-off-by: Sven Strickroth <email@cs-ware.de>