Fix update/updateOrCreate in cloudant.js

[jannyHou]

Use updateAttributes function to update a model instance and use update function in call for updateOrCreate.

Currently, update performs a destructive update. With this change update function should be partial update.

[djorg83]

Possibly related to #110

[jannyHou]

@djorg83 thanks for catching the error in #110 , this is not the same one.
cloudant only support full replace of document, but not partial update.
This issue will implement an update method which does partial update.

[jannyHou]

Some possible solutions:

• Keep it same as replaceOrCreate, and warn users there is not partial update.
• (Tricky) We implement a way to merge data from document and updated data, then call _insert , which will do replace
• Throw error/warning in this method to explicitly tell user they need to define their own method if they do want to partial update.

[cwadding]

@jannyHou one solution for a partial update are to use update handlers https://cloud.ibm.com/docs/services/Cloudant/api?topic=cloudant-design-documents#update-handlers
This would need to be enabled on the cloudant database but can make it so that the updates are not a destructive operation.
It might would be nice to be able to specify the desired update handler