Use instance_exec passing self as an argument for dynamic attributes

Calling `block.call(self)` means any implied scope of block evaluation
goes out the window when an arity of 1 is used. This change uses
`instance_exec`, passing along self as the argument for the block
variable, meaning that the block is still invoked within the context of
the FactoryGirl::Evaluator while allowing the variable to have access to
the evaluator.

Closes #529

lib/factory_girl/attribute/dynamic.rb

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

spec/acceptance/syntax_methods_within_dynamic_attributes_spec.rb

@@ -38,4 +38,14 @@ describe "syntax methods within dynamic attributes" do
     expect(FactoryGirl.build(:post).title).to eq "generate result"
     expect(FactoryGirl.attributes_for(:post)[:title]).to be_nil
   end
+
+  it 'allows syntax methods to be used when the block has an arity of 1' do
+    FactoryGirl.define do
+      factory :post_using_block_with_variable, parent: :post do
+        user {|_| build(:user) }
+      end
+    end
+
+    expect(FactoryGirl.build(:post_using_block_with_variable).user).to be_instance_of(User)
+  end
 end