code: public Attributes put(String key, String value) {
          int i = indexOfKey(key);
          if (i != NotFound)
              vals[i] = value;
          else
              add(key, value);
          return this;
      }
docs: Set a new attribute, or replace an existing one by key.
  @param key case sensitive attribute key
  @param value attribute value
  @return these attributes, for chaining