code: private String getHeaderValue(Response response, String key) throws GitLabApiException {
  
          String value = response.getHeaderString(key);
          value = (value != null ? value.trim() : null);
          if (value == null || value.length() == 0) {
              return (null);
          }
  
          return (value);
      }
docs: Get the specified header value from the Response instance.
  
  @param response the Response instance to get the value from
  @param key the HTTP header key to get the value for
  @return the specified header value from the Response instance, or null if the header is not present
  @throws GitLabApiException if any error occurs