Try to fix issue 3300 with doit()

See the discussion here https://groups.google.com/forum/?fromgroups=#!topic/sympy/5A_XmSZVLf0

I might have broken one test in `sympy/solvers/tests/test_ode.py` which seems to never return any value after I call doit() inside `simplify.py`. 

In `test_nth_linear_constant_coeff_variation_of_parameters_simplify_False()`:
`dsolve(f(x).diff(x, 5) + 2*f(x).diff(x, 3) + f(x).diff(x) -2*x - exp(I*x), f(x), hint + "_Integral", simplify=False)` works fine but `dsolve(f(x).diff(x, 5) + 2*f(x).diff(x, 3) + f(x).diff(x) - 2*x - exp(I*x), f(x), hint + "_Integral", simplify=True)` never terminates.