polarify doesn't descend beneath exprs
support rvs in rvs
check pspace always receives symbol

This is a common fail-point for poorly behaved SymPy Expr code.
density evaluate=False
RandomSymbol._diff_wrt
solvers._ispow checks isinstance Expr
Density(constant) -> Lambda(DiracDelta)
RandomSymbol.is_Atom
given simple case of Eq(Rv, expr) -> subs
ProductContinuousPSpace subs in all symbols in pdf
conjugate prior test
add FiniteDensity(dict)
skip SinglePSpace test_args
dict comprehension to dict(tuple generator)
Merge pull request #1811 from mrocklin/random-parameters

Bugfix in sympy.stats - add support for random parameters