resolveExpression should distinguish string, integer and float

[tonyyang-svail]

As the intermediate representation defines the attribute as map[string]interface{}, the sql package should resolve the following SQLFlow statement

SELECT ...
TRAIN ... 
WITH
    model.objective="multi:softprob",
    train.num_boost_round = 30,
    model.eta = 3.1,
    model.num_class = 3

as string, int, float, int respectively. However, our expression resolver return all these attribute as type string.

[weiguoz]

Shall the resolver consider bool?

[tonyyang-svail]

@weiguoz good question. I think the resolver should consider bool. So train.early_stop = False should be resolved as bool.

[tonyyang-svail]

I wasn't able to modify the resolveExpression because many code-generators are depending on it. So I will add this distinguish functionality as a separate function. After we have migrated all code-generators to the intermediate representation, I will update resolveExpression.

[tonyyang-svail]

Fixed by #865