Respect :unique_index option in #belongs_to

This simply passes the :unique_index passed to
#belongs_to on to ManyToOne#source_key_options

I have no time for refamiliarizing myself with the
testsuite, but it works for our app.

lib/dm-core/associations/many_to_one.rb

@@ -207,6 +207,7 @@ def initialize(name, source_model, target_model, options = {})
  @required = options.fetch(:required, true)
  @key = options.fetch(:key, false)
  @unique = options.fetch(:unique, false)
+ @unique_index = options.fetch(:unique_index, false)
  target_model ||= DataMapper::Inflector.camelize(name)
  options = { :min => @required ? 1 : 0, :max => 1 }.update(options)
  super
@@ -260,10 +261,11 @@ def inverse_name
  # @api private
  def source_key_options(target_property)
  options = DataMapper::Ext::Hash.only(target_property.options, :length, :precision, :scale).update(
- :index => name,
- :required => required?,
- :key => key?,
- :unique => @unique
+ :index => name,
+ :required => required?,
+ :key => key?,
+ :unique => @unique,
+ :unique_index => @unique_index
  )
 
  if target_property.primitive == Integer