Add spec for coercion of nil argument

spec/coercion_of_nil_spec.rb

@@ -0,0 +1,25 @@
+describe "coercion of nil" do
+  before do
+    class Test::Foo
+      extend Dry::Initializer
+      param :bar, proc(&:to_i)
+    end
+
+    class Test::Baz
+      include Dry::Initializer.define -> do
+        param :qux, proc(&:to_i)
+      end
+    end
+  end
+
+  let(:foo) { Test::Foo.new(nil) }
+  let(:baz) { Test::Baz.new(nil) }
+
+  it "works with extend syntax" do
+    expect(foo.bar).to eq 0
+  end
+
+  it "works with include syntax" do
+    expect(baz.qux).to eq 0
+  end
+end