Merchant Invoice Show Page: Update Item Status

[BrianZanti]

As a merchant
When I visit my merchant invoice show page
I see that each invoice item status is a select field
And I see that the invoice item's current status is selected
When I click this select field,
Then I can select a new status for the Item,
And next to the select field I see a button to "Update Item Status"
When I click this button
I am taken back to the merchant invoice show page
And I see that my Item's status has now been updated