Add an update function to the mysql repo.

[igaray]

A straight-up update function, in which you pass a series of fields and values, instead of passing a document.

[elbrujohalcon]

@igaray isn't this already implemented?

[igaray]

No, this is intended to be more flexible and apply changes to more than one document at a time.
I am no longer sure that it is a good idea.

[elbrujohalcon]

oooooh… not a good idea, indeed. Removing it :)