Field is null or undefined when implementing model

[kbarbounakis]

Describe the bug
An error occurred while a data model implements a model which contains tag-like properties e.g.

{
        "@id": "http://schema.org/tags",
        "name": "tags",
        "title": "Tags",
        "description": "Contains a collection of tags for this object.",
        "type": "Text",
        "many": true
      }

https://github.com/themost-framework/data/blob/master/data-model.js#L2504

var tagMapping = inferTagMapping.call(self, field);
            if (tagMapping) {
                //apply data association mapping to definition
                var definitionField = conf.fields.find(function(x) {
                    return x.name === field.name;
                });
                definitionField.mapping = field.mapping = tagMapping;
                return new DataAssociationMapping(definitionField.mapping);
            }

Solution
conf.fields collection may not contain implemented field. So, DataModel.attributes should be used instead.