MEDIUM: http_act: Rework *-headers-bin actions

MEDIUM: http_act: Rework *-headers-bin actions

These actions were added recently and it appeared the way binary headers
were retrieved could be simplified.

First, there is no reason to retrieve a base64 encoded string. It is
possible to rely on the binary string directly. "b64dec" converter can be
used to perform a base64 decoding if necessary.

Then, using a log-format string is quite overkill and probably
conterintuitive. Most of time, the headers will be retrieved from a
variable. So a sample expression is easier to use. Thanks to the previous
patch, it is quite easy to achieve.

This patch relies on the commit "MINOR: action: Add a sample expression
field in arguments used by HTTP actions". The documentation was updated
accordingly.