Update with request data when using validation

[jasonmccreary]

This will generate a statement to fill an existing model with the validated request data on update. Note, this is rather nuanced, only for shorthand references where validation is used.

For example:

update:
    validate: post
    update: post

But not:

update:
    update: post

Or:

update:
    validate: other, columns
    update: name, title, post

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Fixes #292