Fix not implemented error for cache_has_one with embed: false.

@fbogsany for review

## Problem

```ruby
cache_has_one :friends, :embed => false
```

should cause NotImplementedError to be raised.  Instead, it just silently embedded the association.

This was because we had `options[:embed]` before a `if options[:embed]` condition, so the `else` branch was never taken.

## Solution

Use `options[:embed] = true unless options.has_key?(:embed)` to set the default option value.