#update_doc when the doc does not exist is broken

[blairio]

Currently calling doc.update_doc(<id>) with an ID that does not exist throws the wrong exception.

NoMethodError:
       undefined method `[]=' for nil:NilClass
     # ./lib/couchrest/database.rb:254:in `update_doc'

Previous versions of CouchRest threw this exception:

RestClient::ResourceNotFound:` 404 Resource Not Found: {"error":"not_found","reason":"missing"}

Now that #get returns nil, it just carries on and passes nil to the block and then nil to save_doc.

I am not sure if the best solution is to raise an exception such as something like CouchRest:NotFound or pass in a new hash instantiated with the id ie {'_id' => <id>} or something else.

I sort of like instantiating, which I think would be sort of like an upsert.