Identity functions don't handle 'formatted' type

[ChrisLoer]

Adding a text-field that uses an identity function (eg "text-field": { "property": "name", "type": "identity" }) doesn't work after the introduction of the format expression (#6994) because the identity function pathway skips the automatic string->Formatted coercion that the expressions/tokens pathway follows. I think we just need to add an explicit case for Formatted here:

mapbox-gl-js/src/style-spec/function/index.js

Lines 194 to 201 in a1233cb

	function evaluateIdentityFunction(parameters, propertySpec, input) {
	if (propertySpec.type === 'color') {
	input = Color.parse(input);
	} else if (getType(input) !== propertySpec.type && (propertySpec.type !== 'enum' || !propertySpec.values[input])) {
	input = undefined;
	}
	return coalesce(input, parameters.default, propertySpec.default);
	}

/cc @ryanhamley @jplante