`UnicodeDecodeError` when querying index with non-Latin characters

Example (using cyrillic characters in `utf-8`):

```python
index.query('string:*ряж*')
```

causes an `UnicodeDecodeError` error:

```
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd1 in position 8: ordinal not in range(128)
```

A test for this patch added to the file `index_test.py`.