Make sure rack_cache[:verbose] can be set

Before this change, the following config setting which is provided as
example in production.rb of current rails versions:

config.action_dispatch.rack_cache = true

would result in a startup failure.

Fixes #52

lib/lograge.rb

@@ -116,7 +116,8 @@ def self.unsubscribe(component, subscriber)
 
   def self.setup(app)
     self.application = app
-    app.config.action_dispatch.rack_cache[:verbose] = false if app.config.action_dispatch.rack_cache
+    app.config.action_dispatch.rack_cache[:verbose] = false if app.config.action_dispatch.rack_cache &&
+                                                               app.config.action_dispatch.rack_cache.respond_to?(:[]=)
 
     unless app.config.lograge.keep_original_rails_log
       require 'lograge/rails_ext/rack/logger'

spec/lograge_spec.rb

@@ -58,6 +58,43 @@
     end
   end
 
+  describe 'disabling rack_cache verbosity' do
+    subject { -> { Lograge.setup(app_config) } }
+    let(:app_config) do
+      double(config: ActiveSupport::OrderedOptions.new.tap do |config|
+          config.action_dispatch = config_option
+          config.lograge = ActiveSupport::OrderedOptions.new
+          config.lograge.keep_original_rails_log = true
+        end
+      )
+    end
+    let(:config_option) { double(rack_cache: rack_cache) }
+
+    context 'when rack_cache is false' do
+      let(:rack_cache) { false }
+
+      it 'does not change config option' do
+        expect(subject).to_not change { config_option.rack_cache }
+      end
+    end
+
+    context 'when rack_cache is a hash' do
+      let(:rack_cache) { { foo: 'bar', verbose: true } }
+
+      it 'sets verbose to false' do
+        expect(subject).to change { config_option.rack_cache[:verbose] }.to(false)
+      end
+    end
+
+    context 'when rack_cache is true' do
+      let(:rack_cache) { true }
+
+      it 'does not change config option' do
+        expect(subject).to_not change { config_option.rack_cache }
+      end
+    end
+  end
+
   describe 'deprecated log_format interpreter' do
     let(:app_config) do
       double(config: