[FIX] core: compatibility with Werkzeug 0.15 ProxyFix

Werkzeug 0.15 modified ProxyFix such that by default it only forwards
the REMOTE_ADDR when enabled, whereas before 0.15 it would also
forward scheme and host. This breaks proxied odoo as the base url
becomes incorrect (cf #34412).

Use properly configured ProxyFix when running with werkzeug 0.15, old
configuration otherwise.

Backport of 4057227d which was merged
in master, because many people apparently run 0.15 now.

Closes #35085
closes odoo/odoo#36212

closes odoo/odoo#37709

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>