Return ETag header with 304 response

RFC 7232, section 4.1 specifies that when returning a 304 status code the
ETag header must also be included if it would have been sent in a 200 (OK)
response.

httpbin/core.py

@@ -633,7 +633,9 @@ def etag(etag):
 
     if if_none_match:
         if etag in if_none_match or '*' in if_none_match:
-            return status_code(304)
+            response = status_code(304)
+            response.headers['ETag'] = etag
+            return response
     elif if_match:
         if etag not in if_match and '*' not in if_match:
             return status_code(412)

test_httpbin.py

@@ -685,62 +685,71 @@ def test_etag_if_none_match_matches(self):
             headers={ 'If-None-Match': 'abc' }
         )
         self.assertEqual(response.status_code, 304)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_none_match_matches_list(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-None-Match': '"123", "abc"' }
         )
         self.assertEqual(response.status_code, 304)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_none_match_matches_star(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-None-Match': '*' }
         )
         self.assertEqual(response.status_code, 304)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_none_match_w_prefix(self):
         response = self.app.get(
             '/etag/c3piozzzz',
             headers={ 'If-None-Match': 'W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"' }
         )
         self.assertEqual(response.status_code, 304)
+        self.assertEqual(response.headers.get('ETag'), 'c3piozzzz')
 
     def test_etag_if_none_match_has_no_match(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-None-Match': '123' }
         )
         self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_match_matches(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-Match': 'abc' }
         )
         self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_match_matches_list(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-Match': '"123", "abc"' }
         )
         self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_match_matches_star(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-Match': '*' }
         )
         self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.headers.get('ETag'), 'abc')
 
     def test_etag_if_match_has_no_match(self):
         response = self.app.get(
             '/etag/abc',
             headers={ 'If-Match': '123' }
         )
         self.assertEqual(response.status_code, 412)
+        self.assertNotIn('ETag', response.headers)
 
     def test_etag_with_no_headers(self):
         response = self.app.get(