Fix #reload and mark model as persisted after reloading

[andrykonchin]

Fix the following scenario:

• there is a persisted item
• we initialize a new model with the same primary key
• try to save it and fail because such item already exists
• try to reload it

Expected result

• reloaded model should be marked as persisted and #new_record? should return false.

Closes https://github.com/Dynamoid/dynamoid/issues/564