We weren't allowing the `budget_id` parameter and then we were adding it
manually. We were also allowing other parameters that aren't used in the
valuation section.
So we're allowing budget and heading, which are the only parameter we're
offering filters for in the user interface. Note the `budget_id`
parameter doesn't seem to make sense because we're already inside a
`@budget.investments` statement, but the `budget_id` parameter is
required by the `scoped_filter` method.
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