@Override
      public Map<String, String[]> getParameterMap() {
        if (parameters == null) {
          parameters = new HashMap<String, String[]>();
          for (Map.Entry<String, List<String>> entry : m.entrySet()) {
            final List<String> a = entry.getValue();
            parameters.put(entry.getKey(), a.toArray(new String[a.size()]));
          }
        }
        return parameters;
      }
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