Adds a Double decoder.

[ghost]

First of all, what a great library! I really appreciate the work you've done and I'm using it in some of my projects.

I would like to suggest that decoding an Integer value to a Double does not return nil. For example, if I have the following model:

struct Example {
    let value: Double
}

extension Example: Decodable {
    init?(json: JSON) {
        guard let value: Double = "value" <~~ json else { return nil }
        self.value = value
    }
}

and if I try to decode this json and produce a Example model:

{
    "value": 0
}

Gloss will return a nil object, as it was unable to decode an Integer value to a Double. I think that this is due to the fact that 0 isn't recognised as a Decimal value.

A solution to this problem is adding a specific decoder for Doubles.

public static func decode(doubleForKey key: String, keyPathDelimiter: String = GlossKeyPathDelimiter) -> (JSON) -> Double? {
    return {
        json in
            
        if let number = json.valueForKeyPath(keyPath: key, withDelimiter: keyPathDelimiter) as? NSNumber {
            return number.doubleValue
        }
            
        return nil
    }
}

This allows for the key value to be decoded even when the underlying value is an Integer. This can be useful for APIs that send back 0 instead of 0.0.

[hkellaway]

tests need to be added

[hkellaway]

hi @NausJessy - not sure if you noticed the code review request. tests need to be added to this PR.

[ghost]

@hkellaway Sorry, I forgot the tests. I will add them as soon as possible and get back to you.

[hkellaway]

if you have time, it would be good to also have the complement - Encoder update for Double. if not, i'll do when i have a chance

[ghost]

I updated the Encoder as to include the Double complement.

Furthermore, I changed all Double test to use the new decoder and added a case for when an Integer is casted to a Double. I am not very experimented with writing tests, so please feel free to give some feedback about them. If needed, I will change them accordingly.

[adeperio]

Hey guys, thanks for the work on this PR. This is tripping me up on an issue as well. Any idea when you think you'll be pulling this through? Thanks... :)

[hkellaway]

sorry this one got away from me @NausJessy - just rebased and merged manually into develop; so you'll see it in the 2.0.0 release