support polymorphic relationships that use the same AR class

[waymondo]

currently if multiple polymorphic belong_to relationships reference the same class, the dynamic setter method will just use the first match

activerecord-exclusive-arc/lib/exclusive_arc/model.rb

Lines 50 to 55 in e751c0b

	def assign_exclusive_arc(arc, polymorphic)
	attributes = exclusive_arcs.fetch(arc).reflections.to_h do |name, reflection|
	[name, polymorphic.is_a?(reflection.klass) ? polymorphic : nil]
	end
	assign_attributes attributes
	end