Modify one or more existing active budgets to change their limitations or status.
All three types of budgets can be modified.
The following constraints apply when modifying an existing budget.
� campaignIds: a non-empty subset of the original campaign ids MAY be supplied
� amount: an amount MAY be supplied only if the type is not Uncapped and if supplied it MUST be non-negative
� startDate: a future start date MAY be supplied for budgets that have not yet started
� endDate: an end date MAY be supplied and if supplied MUST be a future date greater than the start date
Other attributes MUST NOT be supplied.
Adding new campaigns to a budget is not allowed. In addition, reducing the amount for
a Capped budget to a value less than the current spend not allowed.