Fix Dynamoid::Criteria::Chain#key_present?

[andrykonchin]

Fix issue with Query request.
Any non-equal condition for partition key leads to making Query operation but it should make Scan one.

Example

Document.where(:"id.in" => [])

[coveralls]

Coverage increased (+0.004%) to 97.106% when pulling 639ca76 on andrykonchin:fix-scan-with-condition-on-partition-key into 01f1a3e on Dynamoid:master.

[pboling]

Looks good to me! Thanks for adding a spec!

[richardhsu]

👍 Thanks!