For `asarray` and for the `dtype` equality operator,
equivalent dtype aliases were considered exact matches.
This change ensures that the returned array has a descriptor
that exactly matches the requested dtype.

Note: Intended behavior of `np.dtype('i') == np.dtype('l')`
is to test compatibility, not identity. This change does not
affect the behavior of `PyArray_EquivTypes()`, and the
`__eq__` operator for `dtype` continues to map to
`PyArray_EquivTypes()`.

Fixes numpy#1468.

Use the correct API call to actually set the base object.

Fix a false mismatch. Separate dtype objects, even if equivalent,
cause distinct array views to be created.

* Improve comments/docs.
* Improve descriptiveness of variable names.
* Add additional test expressions that would not pass without
  this patch.

Shorten some lines.