Dimensionless unit looks the same regardless of scale

This comes out of a discussion in #669.

Right now, the following behavior is in astropy:
```
>>> from astropy import units as u
>>> u1 = (u.km / u.m).decompose()
>>> u2 = (u.m / u.m).decompose()
>>> u1
Unit(dimensionless)
>>> u2
Unit(dimensionless)
>>> u1 == u.dimensionless
False
>>> u2 == u.dimensionless
True
>>> u1.is_dimensionless()
True
>>> u2.is_dimensionless()
True
```

This is counter-intuitive, because two things that say they are "dimensionless" give different behavior when compared to ``u.dimensionless``.  

This is "correct" behavior, because two units with different scale factors are different, but the fact that they both appear to be the same is problematic.


cc @mdboom