MINOR: http: split http_transform_header() function in two parts.

MINOR: http: split http_transform_header() function in two parts.

This function is a callback for HTTP actions. This function
creates the replacement string from a build_logline() format
and transform the header.

This patch split this function in two part. With this modification,
the header transformation and the replacement string are separed.

We can now transform the header with another replacement string
source than a build_logline() format.