`zope.index.text.TextIndex` sometimes uses the wrong lexicon

[Alex Smith]

`zope.index.text.TextIndex` will use the wrong lexicon when constructed given an index but no lexicon. In this case, `TextIndex` will use the provided index, but create its own lexicon instead of the lexicon its index is using. `TextIndex` should always use the same lexicon that is used by its index.

At a minimum, `TextIndex` should raise an exception when constructed with an index but no lexicon. Ideally, `TextIndex` would be able to use the lexicon from the provided index itself, but currently there is no specified way to get a lexicon from an index.

[Fred Drake (fdrake)]

It is unfortunate that the constructor for the `TextIndex` class isn't
documented except by the code; the tests cover only the case for
passing no arguments.

As long as there's no API to get a lexicon from the underlying index
that was passed in, the constructor should raise an exception when
passed an index but no lexicon. `ValueError` would be appropriate in
this case.

[Tres Seaver (tseaver)]

http://pypi.python.org/pypi/zope.index/3.6.1