@Override
      public Enumeration<String> getParameterNames() {
        final Iterator<String> i = m.keySet().iterator();
        return new Enumeration<String>() {
          @Override
          public boolean hasMoreElements() {
            return i.hasNext();
          }

          @Override
          public String nextElement() {
            return i.next();
          }
        };
      }
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