BUG/MEDIUM: http: add-header: buffer overwritten

BUG/MEDIUM: http: add-header: buffer overwritten

If we use the action "http-request add-header" with a Lua sample-fetch or
converter, and the Lua function calls one of the Lua log function, the
header name is corrupted, it contains an extract of the last loggued data.

This is due to an overwrite of the trash buffer, because his scope is not
respected in the "add-header" function. The scope of the trash buffer must
be limited to the function using it. The build_logline() function can
execute a lot of other function which can use the trash buffer.

This patch fix the usage of the trash buffer. It limits the scope of this
global buffer to the local function, we build first the header value using
build_logline, and after we store the header name.

Thanks Michael Ezzell for the repporting.

This patch must be backported in 1.6 version