Add value checks to deref tests

[tzlaine]

deref.cpp:

• the test cases in this file only show pass/fail, they   don't indicate the actual type returned by deref, which   will make debugging failures harder.  I typically use   BOOST_MPL_ASSERT((is_same<X, Y>));  specifically for   this reason (even when static_assert is available).
• Actually, in general, these tests need to check both   the type and the value.  Checking just the type is   insufficient.