Simplify service CRUD. The code for update and create was identical.

The only difference was that the update did an `if` block and the create does an inline `if` so it makes sense to just combine them into one method and use an alias for backwards compatibility.

update:

```ruby
unless entity.valid?
  raise Quickbooks::InvalidModelException.new(entity.errors.full_messages.join(','))
end
```

create:

```ruby
raise Quickbooks::InvalidModelException.new(entity.errors.full_messages.join(',')) unless entity.valid?
```