Assert.IsTrue/IsFalse for nullable booleans

[Grauenwolf]

I propose adding the following overloads for handling nullable Booleans:

    public static void IsTrue(bool? condition);
    public static void IsTrue(bool? condition, string message);
    public static void IsTrue(bool? condition, string message, params object[] parameters);
    public static void IsFalse(bool? condition);
    public static void IsFalse(bool? condition, string message);
    public static void IsFalse(bool? condition, string message, params object[] parameters);

They would work exactly like their non-nullable versions.

Reasoning

When checking a Nullable value, we can't use the IsTrue and IsFalse shortcuts. Instead we have to use the longform of Assert.AreEqual( [true|false], which is more error prone because it isn't type safe.

There is also a very minor performance hit with AreEqual because it boxes by default. (You can use AreEqual( (bool?) true, but no one is going to.)