Add support for schemas with type: ["string", "null"]

[ndbroadbent]

Fixes #292

If a schema has type: ["string", "null"] or type: ["null", "integer"], etc., this change will just use the first non-null type from the array.

UPDATE: I just found some other cases where this needs to be handled: I wasn't able to update the value for any optional drop-down select lists (ListField). There might be a few other places with lines of code similar to this:

            if (!self.schema.type || self.schema.type === "string")

These will need to be updated to something like:

            if (!self.schema.type || self.schema.type === "string" || (
                Alpaca.isArray(self.schema.type) && 
                self.schema.type.length === 2 && 
                self.schema.type.indexOf("string") !== -1 && 
                self.schema.type.indexOf("null") !== -1))
            {

(But probably less verbose!)

Maybe there could be a generic "schema type checker" function that checks for any | null.

[renannprado]

Just came across alpaca for the first time and I face exactly this situation at the moment. It would be really nice if this fix is merged :D

[evaisse]

Hye, I've the same problems, could you merge this branch ?