wro4j-core/src/main/java/ro/isdc/wro/cache/CacheKey.java
public CacheKey addAttribute(final String key, final String value) {
    if (key != null && value != null) {
      map.put(key, value);
    }
    return this;
  }
public CacheKey addAttribute(final String key, final String value) {
    if (key != null && value != null) {
      map.put(key, value);
    }
    return this;
  }
Add a custom key-value pair attribute. Each pair is added to an internal map. The custom attributes can be used to
make the key more fine grained (Ex: based on browser version or a request parameter). Both elements of the
attribute (key & value) should be not null. If any of these are null, the attribute won't be added.

@param key
string representing the key of the attribute.
@param value
string representing the value of the attribute.
@return reference to current {@link CacheKey} used for fluent interface.