In case if apache2 fcgid-script handler is being used for RT 6 , I found these following virtualhost settings working ( for passing authorization header to RT )
RewriteEngine On
RewriteCond %{HTTP:Authorization} ^(.*)
RewriteRule .* - [E=HTTP_AUTHORIZATION:%1]
<Directory "/opt/rt/share/html">
AllowOverride All
Options +ExecCGI
Require all granted
</Directory>
<Directory "/opt/rt/sbin">
Require all granted
Options +ExecCGI
SetHandler fcgid-script
</Directory>