UnIndex applies its _convert method on searching but not on indexing

[andbag]

There is another suboptimal thing with UnIndex:
it applies its _convert method on searching but not on indexing.

In general, it is adviceable to process the values for indexing
in the same way as those for searching - otherwise, there is
a risk that searches give surprising results.

The _convert of UnIndex is the identity. Thus, we do not see
a true error. However, if UnIndex would employ _convert homogenously
(both during indexing as well as during searching), then (e.g.)
DateIndex could define its own _convert
and use everything else from UnIndex/FieldIndex.

Originally posted by @d-maurer in #64 (comment)

[andbag]

PR #70 should solve the issue.