@Override
      public String getParameter(String name) {
        final List<String> a = m.get(name);
        return a == null ? null : a.get(0);
      }
The purpose of this function is to get the doAs parameter of a http request case insensitively
@return doAs parameter if exists or null otherwise
hbase-http/src/main/java/org/apache/hadoop/hbase/http/ProxyUserAuthenticationFilter.java