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Merge pull request #15 from rrrene/patch-1

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Remove MassAssignmentSecurity
Don't allow null value for created_at/updated_at fields
Remove #read_attribute

It was redefined in Rails4 and now it's hard to patch it correctly.
It's an internal AR method, so we don't need to implement it in hydra_attribute.
Fix query builder
Set version to 0.6.0.rc1
Fix IdentityMap middleware

Global identity map should be cleared
instead of HydraAttribute::HydraAttribute.identity_map