Using subs() with tuples doesn't work when called as a method

```julia
@syms x, y, z
ex = x + y + z
ex.subs((x, 1), (y, pi))
```
should return `1 + pi + z`, but it returns `x + y + z`. Similarly, `ex.subs((x, 1))` throws an error.

If you write `subs(ex, (x, 1), (y, pi))` instead, it works as expected. I think the method call doesn't dispatch to the correct implementation, so the only method call that works is `ex.subs(var, replacement)`, but I'm not that familiar with Julia, so I'm not sure.

If it's not possible to fix this, the documentation at should be changed to use the `subs(ex, (x, 1), (y, pi))` style instead, and maybe add a note that the method-call syntax doesn't work with tuples.

Thanks for working on this library, it's awesome! :)