mod_asis: Fix the log level of the message AH01236

mod_asis: Fix the log level of the message AH01236

Change the log level from ERROR to DEBUG.

The message \"mod_asis: ap_pass_brigade failed for file ...\" was logged with
the level ERROR. This log level is inappropriate here, because a client can
trigger this log message by aborting the request.

Most other modules don't log at all or use the log level DEBUG when
ap_pass_brigade() fails.

Submitted by: Michael Kaufmann <mail michael-kaufmann.ch>
Github: closes #527

Add a change entry to give credits to the author.

Merges r1924954, r1926191 trunk
Submitted by: jorton, jailletc36
Reviewed by: jailletc36, rpluem, ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1926328 13f79535-47bb-0310-9956-ffa450edef68