Fix Javadoc comments that refer to the actual value under test to refer to "actual value" instead of "subject"

Or more concisely: "`Subject.getSubject()`? Huh?"

I guess that we could make it `Subject.getActualValue()` (or just `getActual()` or `actual()`), which would go nicely with the typical parameter name of `expected`.

Alternatively, we could declare that `FooSubject` should be `FooAsserter` or something. (Apparently [assertj calls this `FooAssert`](http://joel-costigliola.github.io/assertj/assertj-assertions-generator.html), a name that I don't love: What is "an assert?")