public A header(String key, String value) {
    headers.put(key, value);
    return getInstance();
  }
Sets the header named {@code key} to {@code value}. If this request already has any headers
with the same key, the value is replaced.

@param key the header key
@param value the header value
@return this request
src/main/java/com/google/maps/PendingResultBase.java