improved schema preprocessor and Date object handling (validation/serialization) for response bodies

[cdimascio]

see #493 for a detailed discussion on this topic

• resolves Response validation: Automatically coerce date data type to ISO string #288 and Date is not validated correctly as type: string, format: date-time #246
• partially resolves Serialize in request validation and authorize object when toJSON() matches to the format #353 - solves for dates, but not mongodb ObjectId. it does pave the way for a custom serializer for MongoDb ObjectId
• paves the way for How do you handle mongodb ObjectId ? #465

[electrotype]

@cdimascio I didn't validate the code itself, but I did validate this branch in my project and it works #1, without any modification required on my side.

Fantastic work!!

[electrotype]

This is the magical line, isn't it?

(schema.type = 'object'), (schema.schemaObjectFunctions = dateTime);

:-)