MINOR: http: don't %-encode the payload when not relevant

MINOR: http: don't %-encode the payload when not relevant

As reported by Pierre Maoui in GH #2477, it's not possible to render
control chars from variables or expressions verbatim in the payload part
of http-return statements. That's a problem because this part should not
require to be encoded at all (we could even imagine building favicons on
the fly for example).

In fact it is the LOG_OPT_HTTP option when passed as default options on
parse_logformat_string() which tells the log encoder that the payload
should be http-encoded using lf_chunk() instead of being printed using the
per-type encoder.

This option was set when parsing logformat expressions for lf-string
expression under http-return statements, as well as logformat expressions
for set-map action. While it is true that those actions may only be
used under http context, the LOG_OPT_HTTP logformat option is not relevant
there, because the payload is expected to be used without being encoded.

So let's simply get rid of this option when parsing logformat expressions
for set-map action key/value and lf-string from http-request return
action, and add a note next to LOG_OPT_HTTP option to indicate that it is
used to tell the log encoder that the payload should be HTTP-encoded.

Thanks to Pierre for having reported the issue and Willy for the
analysis and patch proposal.