Tried to rectify bug in solve_linear

Hi, solve_linear gives me the following results in master, which I think is wrong,

    eq = y - Integral(f(x), y)
    solve_linear(eq)
    (y, Integral(f(x), y)/(-f(x) + 1))
    eq = x + y - Integral(f(x), y)
    solve_linear(eq)
    (y, -x/(-f(x) + 1) + Integral(f(x), y)/(-f(x) + 1))

I tried to rectify it but there are a couple of test failures. It would be really nice if someone could help me out and merge this, since the source code of `solvers.py` is relatively new to me.