correctly initialize env and headers

lib/grape/router.rb

@@ -136,6 +136,13 @@ def method_not_allowed(env, methods, endpoint)
       env[Grape::Env::GRAPE_METHOD_NOT_ALLOWED] = true
       current = endpoint.dup
       current.instance_eval do
+        @env = env
+        @header = {}
+
+        @request = Grape::Request.new(env)
+        @params = @request.params
+        @headers = @request.headers
+
         @lazy_initialized = false
         lazy_initialize!
         run_filters befores, :before

spec/grape/api_spec.rb

@@ -556,6 +556,20 @@ def subject.enable_root_route!
       end
     end
 
+    context 'when accessing env' do
+      it 'returns a 405 for an unsupported method' do
+        subject.before do
+          custom_header = env['HTTP_X_CUSTOM_HEADER']
+        end
+        subject.get 'example' do
+          'example'
+        end
+        put '/example'
+        expect(last_response.status).to eql 405
+        expect(last_response.body).to eql '405 Not Allowed'
+      end
+    end
+
     specify '405 responses includes an Allow header specifying supported methods' do
       subject.get 'example' do
         'example'