add BigDecimal conversion for formatted currency strings

lib/rails_param/param.rb

@@ -56,6 +56,7 @@ module RailsParam
         return Array(param.split(options[:delimiter] || ",")) if type == Array
         return Hash[param.split(options[:delimiter] || ",").map{|c| c.split(options[:separator] || ":")}] if type == Hash
         return (/(false|f|no|n|0)$/i === param.to_s ? false : (/(true|t|yes|y|1)$/i === param.to_s ? true : nil)) if type == TrueClass || type == FalseClass || type == :boolean
+        return BigDecimal.new(param.delete('$,').strip) if type == BigDecimal && param.is_a?(String)
         return BigDecimal.new(param, (options[:precision] || DEFAULT_PRECISION)) if type == BigDecimal
         return nil
       rescue ArgumentError

spec/rails_param/param_spec.rb

@@ -112,6 +112,13 @@ describe RailsParam::Param do
           controller.param! :foo, BigDecimal, precision: 6
           expect(controller.params["foo"]).to eql 12345.7
         end
+
+        it "converts formatted currency string to big decimal" do
+          allow(controller).to receive(:params).and_return({ "foo" => "$100,000"})
+          controller.param! :foo, BigDecimal
+          expect(controller.params["foo"]).to eql 100000.0
+        end
+
       end
 
       describe "booleans" do