Piecewise._eval_interval for cond. indep. of integration var.

This fixes the definite Integral of a Piecewise when the conditions are independent of the integration variable.

Before:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
4*y/sqrt(y**2)

After:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
Piecewise((0, y == 0), (4*y/sqrt(y**2), True))

Fixes issue 3648
http://code.google.com/p/sympy/issues/detail?id=3648