Simplify header merging

qwarc/__init__.py

@@ -52,13 +52,7 @@ class Item:
 	def _merge_headers(self, headers, extraHeaders = []):
 		d = {}  # Preserves order from Python 3.7 (guaranteed) or CPython 3.6 (implementation detail)
 		keys = {}  # casefolded key -> d key
-		for key, value in self.headers:
-			d[key] = value
-			keys[key.casefold()] = key
-		for key, value in extraHeaders:
-			d[key] = value
-			keys[key.casefold()] = key
-		for key, value in headers:
+		for key, value in itertools.chain(self.headers, extraHeaders, headers):
 			keyc = key.casefold()
 			if value is None:
 				if keyc in keys: