Improve DatabaseQuery.input custom support

[valeriomazzeo]

Because of some assumptions made in Querybuilder+Set and Querybuilder it's not possible to assign anything other than DatabaseQuery.Value.dictionary as a top level Input. In particular it prevents the use of .custom.

An example of how to circumvent the current limitation is visible here. Unfortunately, this rely on the specific implementation details from Fluent, and I suspect it may break at any time.

Expected behaviour would be to be able to extend Querybuilder.set or more in general, to be able to freely manipulate the Input array without worrying that Fluent will either crash or replace its content completely.