net: http_client: Ignore message body on 101 Switching Protocols reply

When HTTP 101 Switching Protocols response is received, force the HTTP
parser to ignore any potential message body payload (which should
already belong to the new protocol).

This is usually not an issue, as Switching Protocols reply should
contain headers only, however it's been observed that some servers
specify chunked transfer-encoding header which tricks the parser
to wait for payload event though it never arrives.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>