apreq_header_attribute: Search for the exact attribute name.

Improve the parsing of the header attributes such that we don't match any
special character before that attribute name (e.g. "(boundary=") or let
forbidden characters unnoticed.



git-svn-id: https://svn.apache.org/repos/asf/httpd/apreq/trunk@1894982 13f79535-47bb-0310-9956-ffa450edef68