code: public String getValue(final String key) {
          Validate.notEmpty(key, "key cannot be empty");
          
          final List<String> allValues = this.getValues(key);
          return allValues != null ? allValues.get(0) : null;
      }
docs: Returns the first value for the given key
  @param key key (case insensitive)
  @return single (first) value for the given key or {@code null}, if there is no such a key in this instance