Fix saving record using #update_attributes

Change-Id: I8e775dead0a610ff610aa66834524cb661339596
Reviewed-on: http://review.couchbase.org/41273
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Allow save & save! without performing validations

Fixes #17

Change-Id: I2dad010c8739bf7b0e81b02f449877edaa26a16b
Reviewed-on: http://review.couchbase.org/41272
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Merge branch 'master' of https://github.com/stakach/couchbase-ruby-model