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: I7d23321fc8c2e982517aabb419cdf9a29c57228d
Use context return for simple is_many accessors.
Allow updating is_many with single value.
Test for getting/setting is_many with one or many values.
Update test to also test indirect creation.
Merge pull request #17 from tmooney/master