Check arity to allow for Symbol#to_proc with dynamic attributes

Closes #698

lib/factory_girl/attribute/dynamic.rb

@@ -11,7 +11,10 @@ module FactoryGirl
         block = @block
 
         -> {
-          value = block.arity == 1 ? instance_exec(self, &block) : instance_exec(&block)
+          value = case block.arity
+                  when 1, -1 then instance_exec(self, &block)
+                  else instance_exec(&block)
+                  end
           raise SequenceAbuseError if FactoryGirl::Sequence === value
           value
         }

spec/acceptance/definition_spec.rb

@@ -19,3 +19,36 @@ describe "an instance generated by a factory with a custom class name" do
   it { should be_admin }
 end
 
+describe "attributes defined using Symbol#to_proc" do
+  before do
+    define_model("User", password: :string, password_confirmation: :string)
+
+    FactoryGirl.define do
+      factory :user do
+        password "foo"
+        password_confirmation &:password
+      end
+    end
+  end
+
+  it "assigns values correctly" do
+    user = FactoryGirl.build(:user)
+
+    expect(user.password).to eq "foo"
+    expect(user.password_confirmation).to eq "foo"
+  end
+
+  it "assigns value with override correctly" do
+    user = FactoryGirl.build(:user, password: "bar")
+
+    expect(user.password).to eq "bar"
+    expect(user.password_confirmation).to eq "bar"
+  end
+
+  it "assigns overridden value correctly" do
+    user = FactoryGirl.build(:user, password_confirmation: "bar")
+
+    expect(user.password).to eq "foo"
+    expect(user.password_confirmation).to eq "bar"
+  end
+end