Further tests for String.Concat and String.Join

[jamesqo]

This increases the test coverage for string.Concat and string.Join by calling Join<T> and Concat<T> with a type parameter of string, as well as adding tests for a couple of other edge cases..

cc @hughbe @stephentoub

[stephentoub]

This comment isn't necessary.

[stephentoub]

One nit, otherwise LGTM. Thanks.

[jamesqo]

I ran into a few quirks including the Join(string, object[]) bug where it will return null if the first item is null, and another one where Concat(object) returns null if the ToString is null. Have updated the tests to account for these, everything should pass now.

[stephentoub]

LGTM. Thanks for the additions.