Add validation of selection in DictSchemaMixin
Traceback (most recent call last):
File "/trytond/protocols/jsonrpc.py", line 123, in _marshaled_dispatch
response['result'] = dispatch_method(method, params)
File "/trytond/protocols/jsonrpc.py", line 156, in _dispatch
res = dispatch(*args)
File "/trytond/protocols/dispatcher.py", line 156, in dispatch
result = rpc.result(meth(*args, **kwargs))
File "/trytond/model/dictschema.py", line 74, in get_keys
selection = dict(json.loads(english_key.selection_json))
ValueError: dictionary update sequence element #0 has length 1; 2 is required