Don't enable evented mode unless gevent is enabled.
Previous code was activating evented mode as soon as gevent is imported which
may happen in different scenario but is not enough to assume the GeventServer
is actually running.
Closes#13463
According to RFC 7232 # 2.3, an etag must be wrapped in double quotes:
entity-tag = [ weak ] opaque-tag
opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
Odoo didn't properly quote etags, which could lead to stripping or
failures when putting Odoo behind strict HTTP proxies.
andreparames