fix test case for duplicated tracestate keys

The test_tracestate_duplicated_keys was broken, it demanded to drop
the whole tracestate when a duplicate key was found. This does not match
what the specification mandates in that situation. Instead,
implementations MAY remove the duplicates from tracestate or even leave
them in as-is.

fixes #369

test/test.py

@@ -549,37 +549,38 @@ def test_tracestate_multiple_headers_different_keys(self):
 	def test_tracestate_duplicated_keys(self):
 		'''
 		harness sends a request with an invalid tracestate header with duplicated keys
-		expects the tracestate to be discarded
+		expects the tracestate to be inherited, and the duplicated keys to be either kept as-is or one of them
+        to be discarded
 		'''
 		traceparent, tracestate = self.make_single_request_and_get_tracecontext([
 			['traceparent', '00-12345678901234567890123456789012-1234567890123456-00'],
 			['tracestate', 'foo=1,foo=1'],
 		])
 		self.assertEqual(traceparent.trace_id.hex(), '12345678901234567890123456789012')
-		self.assertRaises(KeyError, lambda: tracestate['foo'])
+		self.assertTrue('foo=1' in str(tracestate))
 
 		traceparent, tracestate = self.make_single_request_and_get_tracecontext([
 			['traceparent', '00-12345678901234567890123456789012-1234567890123456-00'],
 			['tracestate', 'foo=1,foo=2'],
 		])
 		self.assertEqual(traceparent.trace_id.hex(), '12345678901234567890123456789012')
-		self.assertRaises(KeyError, lambda: tracestate['foo'])
+		self.assertTrue('foo=1' in str(tracestate) or 'foo=2' in str(tracestate))
 
 		traceparent, tracestate = self.make_single_request_and_get_tracecontext([
 			['traceparent', '00-12345678901234567890123456789012-1234567890123456-00'],
 			['tracestate', 'foo=1'],
 			['tracestate', 'foo=1'],
 		])
 		self.assertEqual(traceparent.trace_id.hex(), '12345678901234567890123456789012')
-		self.assertRaises(KeyError, lambda: tracestate['foo'])
+		self.assertTrue('foo=1' in str(tracestate))
 
 		traceparent, tracestate = self.make_single_request_and_get_tracecontext([
 			['traceparent', '00-12345678901234567890123456789012-1234567890123456-00'],
 			['tracestate', 'foo=1'],
 			['tracestate', 'foo=2'],
 		])
 		self.assertEqual(traceparent.trace_id.hex(), '12345678901234567890123456789012')
-		self.assertRaises(KeyError, lambda: tracestate['foo'])
+		self.assertTrue('foo=1' in str(tracestate) or 'foo=2' in str(tracestate))
 
 	def test_tracestate_all_allowed_characters(self):
 		'''

test/tracecontext/tracestate.py

@@ -56,9 +56,11 @@ def from_string(self, string):
 				if not match:
 					raise ValueError('illegal key-value format {!r}'.format(member))
 				key, eq, value = match.groups()
-				if key in self._traits:
-					raise ValueError('conflict key {!r}'.format(key))
-				self._traits[key] = value
+				if key not in self._traits:
+					self._traits[key] = value
+					# If key is already in self._traits, the incoming tracestate header contained a duplicated key.
+					# According to the spec, two behaviors are valid: Either pass on the duplicated key as-is or drop
+					# it. We opt for dropping it.
 		return self
 
 	def to_string(self):