Correct behavior of HTTP request headers sent by ab in presence of -H command-

Correct behavior of HTTP request headers sent by ab in presence of -H command-
line overrides.  Previously, ab would concatenate a supplied -H User-Agent:
header to the existing one, and send duplicate headers if either -H Host: or
-H Accept: were specified on the command line.

Now, the default headers are not sent if they are overridden using the -H
command-line flag.

Submitted by:  Arvind Srinivasan arvind.srinivasan  sun.com
Reviewed by: sctemme
PR: 31268, 26554

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@526872 13f79535-47bb-0310-9956-ffa450edef68