Correct ASCII-to-Unicode readback for attribute_filter

[johnkerl]

Context: #106

See also: #138 #101 #99

The idea is that:

• For the next few months, pending an upcoming TileDB-core release, we'll be taking obs/var Unicode strings and storing them as "ASCII" which really means bytes -- "α,β,γ" stores as "\xce\xb1,\xce\xb2,\xce\xb3"
• On readback of obs/var dataframes we decode back to "α,β,γ"
• This was being done correctly on the AnnotationDataFrame's dim_select / df accessor; the same logic needs applied to its attribute_filter accessor