Can't `Rule.test` scalar data

[aplowman]

If we have a condition: cnd = Value.dtype.equal_to(str), we can test a datum like this: cnd.test('hello') -> bool. But if we have a rule with an empty path argument:

rule = Rule(path=[], condition=cnd)

...we cannot test a scalar datum in the same way:

rule.test('hello') # raises TypeError

...because we cast "hello" to a Data object, which expects a list or dict.

The workaround is to wrap the datum in a list and set the path to the first list element, like this:

rule = Rule(path=[0], condition=cnd)
rule.test(["hello"]) # success