Updated TableHelperExtensionMethods.FillInstance() to populate fields in addition to properties

A limitation of TableHelperExtensionMethods.FillInstance() is it will only operate on properties. There are cases where it would be nice if it populated public fields as well. For example. SvcUtil.exe, a utility for generating service proxies, generates message contracts with fields instead of properties. It would be very helpful to be able to populate these message contracts using the table helper methods.