2440: Now equal Integrals compare equal when using different variables.

Now we have that

    >>> Integral(x, (x, 0, 1)) == Integral(y, (y, 0, 1))
    True
    >>> Integral(x, (x, 0, 1)) + Integral(y, (y, 0, 1))
    2*Integral(x, (x, 0, 1))

Although it still does not take into account different ordering of
variables in multiple integrals (issue 2297):

    >>> Integral(x*y**2, (x, 0, 1), (y, 0, 2)) == Integral(y*x**2,
    ...     (y, 0, 1), (x, 0, 2))
    True
    >>> Integral(x*y**2, (x, 0, 1), (y, 0, 2)) == Integral(y*x**2,
    ...     (x, 0, 2), (y, 0, 1))
    False