Change MultiValueMap to extend Map<K, List<? extends V>>

[jonfreedman]

If we change MultiValueMap to wildcard the List type it extends then this code will compile

final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<>();
final  List<String> enumFormStringArray = Collections.singletonList("foo");
formParams.put("enum_form_string_array", enumFormStringArray);

This will allow us to clear up some warnings in the openapi-generator (OpenAPITools/openapi-generator#5782)

[jonfreedman]

I had a look at this change, it seems to me that the #addAll method was added to solve this, although the semantics are slightly different. I think what really should happen is that this interface should not extend Map at all, similar to the Multimap interface in Guava.

[snicoll]

Thanks for the suggestion but we'd like to retain the same contract as what java.util.Map does.