Empty accelerated fields leads to TypeError #677

Describe the bug

def __getitem__(self, item):
        res = Collection.__getitem__(self, item)
        for k, v in res.content.items():
            if "accelerated_fields" in k:
                res.content[k] = json.loads(v)
        return res

This code breaks if v is not a valid JSON string.
As there is no way to remove an acceleration other than setting it to empty, this is a typical case.

To Reproduce
Steps to reproduce the behaviour:

c.kvstore.create("test")
collection = c.kvstore["test"]
collection.update_accelerated_field("test", {"test": 1})
collection = c.kvstore["test"]
collection.update_accelerated_field("test", "")
collection = c.kvstore["test"]

Expected behaviour
Rather than an exception, the empty string is used as the value.

If I am allowed to dream, splunkd should provide a mechanism to remove accelerations and field configurations cleanly. However, this bug should still be fixed, as we never know what administrators might write in .conf files.