ap_rset_content_type() should take const char * instead of char *

this is friendlier to callers and r->content_type is const char *
too so it isn't harmful

this fixes a fatal compile error with AIX+xlc


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94051 13f79535-47bb-0310-9956-ffa450edef68