Fixed bug in _integer_basis; added test.
Expanded the docstring of _integer_basis.

``_integer_basis`` determines the scaling factor to put an algebraic
equation in simpler form; a bug in it has been fixed in this PR.

For example in the following example there is no scaling factor

```
>>> from sympy.polys import Poly
>>> from sympy.abc import x
>>> Poly(x**5 - 40*x + 64, x, domain='ZZ').all_roots()[0]
RootOf(x**5 - 40*x + 64, 0)
```

while in master one gets scaling factor 2, and wrong roots.
corrected docstring in ``_integer_basis``
Merge pull request #1938 from pernici/roots1

Fixed bug in _integer_basis