Auto-box single values for is_many properties in creation.

This creates more symmetry between get() and create() when passing
single values for an is_many property.

Change-Id: I7e88c164259c699f5157b7c2460ff268f0cf8d04
Revert "Auto-box single values for is_many properties in creation."

This reverts commit 823e09a0e513a06e434e812495e9c2f62692687c.