Make search with filtering match partial_search.

Fixes https://github.com/opscode/chef/issues/2679.
i think you could do this:

```
attr_accessor :rest
attr_accessor :config

def initialize(url=nil, config: Chef::Config)
  @config = config
end

def rest
  @rest = Chef::REST.new(url || config[:chef_server_url])
end
```

not sure if i got my ruby-2.0 syntax right, but lazying the rest object and allowing DI'ing the config class.