allow reseting of request variants

The current implementation of `variants=` don't allow a resetting to nil, wich is the default value.

This results in the following code smell:
```ruby
case request.user_agent
when /iPhone/
  request.variants = :phone
when /iPad/
  request.variants = :ipad
end
```

With the ability to reset variants to nil, it could be:
```ruby
request.variants = case request.user_agent
when /iPhone/
  :phone
when /iPad/
  :ipad
end
```

1 files changed, 7 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/request_test.rb b/actionpack/test/dispatch/request_test.rb
index beb9085abe..ee8e915610 100644
--- a/actionpack/test/dispatch/request_test.rb
+++ b/actionpack/test/dispatch/request_test.rb
@@ -1143,6 +1143,13 @@ class RequestVariant < BaseRequestTest
     end
   end
 
+  test "reset variant" do
+    request = stub_request
+
+    request.variant = nil
+    assert_equal nil, request.variant
+  end
+
   test "setting variant with non symbol value" do
     request = stub_request
     assert_raise ArgumentError do