ActiveRemote::Base#save produces false negative on additional calls

When creating or updating a record with ActiveRemote you can get a false negative with the save command.

```ruby
user  = User.new
user.save # => false
user.errors # => [ "Need an email" ]

user.email = "foo@bar.com"
user.save # => false
user.errors # => [ "Need an email" ]
```

The contrived example above shows a workflow for producing the issue:

1. Use `.new` to instantiate an unsaved object.
2. Save the object with `#save` producing an error from the service.
3. The errors list is populated with information about the failure.
3. Correct the error by applying the correct attributes.
4. Save the object again. `#save` returns false because the errors list from the previous issue was not cleared on the save.

I'll submit a pull request for it sometime in the next few days, but just wanted to document to make sure I remember what happened.