[ios, macos] Nested attributes does not get parsed according to the expressions spec.

[fabian-guerra]

Given the following JSON:

"properties":{
            "lineStyle":{
                "color":"#00ffff",
            },
}    

Trying to access the lineColor attribute using the following expression
[NSExpression expressionForKeyPath:@"lineStyle.color"] should parse into
@[@"get", @"color", @[@"get", @"lineStyle"]] but gets parsed as @[@"get", @"lineStyle.color"] instead.

/cc @anandthakker @jmkiley @1ec5

[1ec5]

It looks like we don’t handle the valueForKeyPath: function’s argument like a key path:

mapbox-gl-native/platform/darwin/src/NSExpression+MGLAdditions.mm

Lines 1064 to 1065 in a1195f6

	} else if ([function isEqualToString:@"valueForKey:"] || [function isEqualToString:@"valueForKeyPath:"]) {
	return @[@"get", self.arguments.firstObject.mgl_jsonExpressionObject, self.operand.mgl_jsonExpressionObject];

However, I’m unsure how this code for key path expressions is incorrect:

mapbox-gl-native/platform/darwin/src/NSExpression+MGLAdditions.mm

Lines 1046 to 1056 in a1195f6

	case NSKeyPathExpressionType: {
	NSArray *expressionObject;
	for (NSString *pathComponent in self.keyPath.pathComponents.reverseObjectEnumerator) {
	if (expressionObject) {
	expressionObject = @[@"get", pathComponent, expressionObject];
	} else {
	expressionObject = @[@"get", pathComponent];
	}
	}
	return expressionObject;
	}