data-prototype does not accurately reflect the contents of the attribute. I propose the following attributes on collections for Symfony 3.0.

• data-form-type='collection' - label this form widget as a collection
• data-collection-prototype - Stores the collection prototype string
• data-collection-prototype-name - The prototype name. Used for label replacement.
• data-collection-allow-add - The collection allows prototypes to be added
• data-collection-allow-delete - The collection allows prototypes to be deleted

See: #5145 & #7713