Improved RangedNumberGenerator robustness for heterogeneous requests

In scenarios with multiple heterogeneous RangedNumberRequests,

 1. a request can be of type System.Byte
 2. the next request can be of a type with maximum value greater than System.Byte.MaxValue
 3. and then, the next request can be (again) of type System.Byte – *then* the RangedNumberGenerator state *could* be greater than System.Byte.MaxValue and a System.OverflowException was thrown, as reported in #295.
Agreed, I think it's OK to add a case for Int16, although such overflow is quite rare to occur – unless someone intentionally tries to achieve it e.g. with a shared Fixture instance, a *for 1 ... 1000 loop* and a big object graph :)