Fix shoulda-matchers in Rails 5

Backports a fix for https://github.com/thoughtbot/shoulda-matchers/issues/913.
This can be removed once new shoulda-matchers version is released.

spec/support/shoulda/matchers/rails_shim.rb

+# monkey patch which fixes serialization matcher in Rails 5
+# https://github.com/thoughtbot/shoulda-matchers/issues/913
+# This can be removed when a new version of shoulda-matchers
+# is released
+module Shoulda
+  module Matchers
+    class RailsShim
+      def self.serialized_attributes_for(model)
+        if defined?(::ActiveRecord::Type::Serialized)
+          # Rails 5+
+          serialized_columns = model.columns.select do |column|
+            model.type_for_attribute(column.name).is_a?(
+              ::ActiveRecord::Type::Serialized
+            )
+          end
+
+          serialized_columns.inject({}) do |hash, column| # rubocop:disable Style/EachWithObject
+            hash[column.name.to_s] = model.type_for_attribute(column.name).coder
+            hash
+          end
+        else
+          model.serialized_attributes
+        end
+      end
+    end
+  end
+end