Fix nil strategy assignment for non-default databases

lib/database_cleaner/cleaner.rb

@@ -49,7 +49,7 @@ def strategy=(args)
         raise ArgumentError, "You must provide a strategy object, or a symbol for a known strategy along with initialization params."
       end
 
-      set_strategy_db @strategy, db
+      set_strategy_db self.strategy, db
     end
 
     def strategy

spec/database_cleaner/cleaner_spec.rb

@@ -154,6 +154,14 @@ module DatabaseCleaner
         cleaner.strategy = :truncation, { param: "one" }
       end
 
+      it "accepts nil as the null strategy for a non-default database" do
+        cleaner.db = :test_db
+        cleaner.strategy = nil
+
+        expect(cleaner.strategy).to be_a(DatabaseCleaner::NullStrategy)
+        expect(cleaner.cleaning { :result }).to eq :result
+      end
+
       it "should accept strategy objects" do
         strategy = double
         cleaner.strategy = strategy

spec/database_cleaner/cleaners_spec.rb

@@ -53,6 +53,16 @@
     end
   end
 
+  it "sets the null strategy across default and non-default databases" do
+    cleaners[:active_record]
+    cleaners[:active_record, db: :second_db]
+
+    cleaners.strategy = nil
+
+    expect(cleaners.strategy.values).to all(be_a(DatabaseCleaner::NullStrategy))
+    expect(cleaners.cleaning { :result }).to eq :result
+  end
+
   context "top level api methods" do
     context "single orm single db" do
       let!(:cleaner) { cleaners[:active_record] }