check if inferred type is interfrace, not declared type

original code creates proxy even if inferred type is class

We had code like this:
[SitecoreChildren(IsLazy = false, InferType = true)]
public virtual IEnumerable<IGroup> ChildrenGroups { get; protected set; }

and had several classes/items implementing IGroup. All those classes are decorated with template ids so with "IsLazy = false, InferType = true" I expected to get instances of my classes but I got proxies instead. Proposed change fixes the problem and everything works as expected.