follow-up #511

An exception occurs when invoking the rake task via ./bin/rails.

```
bin/rails aborted!
NoMethodError: undefined method `watch' for Spring:Module (NoMethodError)

    Spring.watch event.payload[:env].filename if Rails.application
          ^^^^^^
.bundle/ruby/3.2.0/gems/dotenv-3.1.3/lib/dotenv/rails.rb:16:in `block in <main>'
```

When using `./bin/rails`, 'spring/client' is loaded, but 'spring/watcher', which is unnecessary for the client-side, is not required.

This PR follows up on #511 and fixes exception.

lib/dotenv/rails.rb

@@ -11,7 +11,7 @@
 
 # Watch all loaded env files with Spring
 ActiveSupport::Notifications.subscribe("load.dotenv") do |*args|
-  if defined?(Spring)
+  if defined?(Spring) && Spring.respond_to?(:watch)
     event = ActiveSupport::Notifications::Event.new(*args)
     Spring.watch event.payload[:env].filename if Rails.application
   end

spec/dotenv/rails_spec.rb

@@ -82,6 +82,14 @@
     expect(Spring.watcher).to include(path.to_s)
   end
 
+  it "doesn't raise an error if Spring.watch is not defined" do
+    stub_spring(load_watcher: false)
+
+    expect {
+      application.initialize!
+    }.to_not raise_error
+  end
+
   context "before_configuration" do
     it "calls #load" do
       expect(Dotenv::Rails.instance).to receive(:load)
@@ -206,13 +214,19 @@
     end
   end
 
-  def stub_spring
-    spring = Struct.new("Spring", :watcher) do
-      def watch(path)
+  def stub_spring(load_watcher: true)
+    spring = Module.new
+
+    if load_watcher
+      def spring.watcher
+        @watcher ||= Set.new
+      end
+
+      def spring.watch(path)
         watcher.add path
       end
     end
 
-    stub_const "Spring", spring.new(Set.new)
+    stub_const "Spring", spring
   end
 end