BUG/MINOR: log/encode: consider global options for key encoding

BUG/MINOR: log/encode: consider global options for key encoding

In sess_build_logline(), contrary to what's stated in the comment
"only consider global ctx for key encoding", we check for
LOG_OPT_ENCODE flag on the current ctx options instead of global
ones. Because of this, we could end up doing the wrong thing if the
previous node had encoding enabled but it isn't set globally for
instance.

To fix the issue, let's simply check the presence of the flag on
g_options before entering the "key encoding" block.

This bug was introduced with 3f7c8387 ("MINOR: log: add +json encoding
option"), no backport needed.