Allow IsOfType to accept subclasses of the specified type

[mysticfall]

Currently, IsOfType throws an exception when type of the passed object is different from the specified type.

However, it is often desired to see if an object is a subclass of a certain type. So, it could be useful if we can make IsOfType optionally accept any subclass of the specified type rather than an instance that exactly matches it.

[danielwertheim]

@mysticfall To not break existing functionality, would it work if a new method was introduced, e.g IfAnyTypeOf ?

[mysticfall]

Sure, it sounds like a reasonable thing to do, preserving compatibility for existing users to me.

[ndrwrbgs]

I've implemented this in https://www.nuget.org/packages/Ensure.That.Experimental/0.1.0 since this Ensure.That library provides a really nice extendable API (at least for the Ensure.That and Ensure.{Type} syntaxes).

I'd like to move that under @danielwertheim's account or into this library, as applicable, but keeping it separate meant I didn't have to try to match existing style/patterns (okay let's be honest, I mean I didn't write tests yet)

[danielwertheim]

I'll gladly accept any PR that is up-to date and that follows the API (so that not yet another flavor is added, unless a good case could be provided for why) and has some (at least one) test.

[loedeman]

@danielwertheim Just did that - I hope this issue can be solved soon :).