Prevent users from mutating the extra_params hash

Instead they should always use the `extra_params=` writer so that the
path info gets updated.

Author: Sam Phippen

lib/rspec/rails/view_spec_methods.rb

@@ -39,6 +39,7 @@ def extra_params=(hash)
           #     # => { :id => 4 }
           def extra_params
             @extra_params ||= {}
+            @extra_params.dup.freeze
           end
         end
       end

spec/rspec/rails/view_spec_methods_spec.rb

@@ -16,6 +16,16 @@ class ::VCSampleClass; end
           expect(VCSampleClass.instance_methods.map(&:to_sym)).to(include(:extra_params=))
           expect(VCSampleClass.instance_methods.map(&:to_sym)).to(include(:extra_params))
         end
+
+        describe "the added #extra_params reader" do
+          it "raises an error when a user tries to mutate it" do
+            ViewSpecMethods.add_to(VCSampleClass)
+
+            expect {
+              VCSampleClass.new.extra_params[:id] = 4
+            }.to raise_error(RuntimeError)
+          end
+        end
       end
 
       describe "when accessors are already defined" do