Fix issue with unicode json path keys

[jjjacksn]

This fixes an issue with unicode keys in JSON path. I observed the bug on Python 2.7.11.

The previous implementation of SQLBuilder.eval_json_path() did not consider unicode objects valid keys.

Here is some sample code which should reproduce the issue.

from pony.orm import *

db = Database()

class MyEntity(db.Entity):
    id = PrimaryKey(int, auto=True)
    json = Optional(Json

...

def query_json():
    with db_session:
        my_entities = select(e for e in MyEntity if e.json[u'key'] == 'value')[:]

    return my_entities

I ran into this issue because I am developing a Python 2/3 codebase using the unicode literals feature from future. The workaround I am using for now is as follows:

from __future__ import unicode_literals

import sys
PY2 = sys.version_info[0] == 2

...

def _json_key(key):
    return bytes(key) if PY2 else key

def query_json():
    with db_session:
        my_entities = select(e for e in MyEntity if e.json[_json_key('key')] == 'value')[:]

    return my_entities

[kozlovsky]

Thanks for reporting! I fixed it in a slightly different way, and covered it with a test

[jjjacksn]

Great thanks!