Enhance parsing of Accept-Language quality values

The following cases were not possible:
"Accept-Language: en-US; foo=bar; q=0.9"
"Accept-Language: en-US; q = 0.9"

tornado/test/web_test.py

@@ -3183,6 +3183,14 @@ def test_accept_language(self):
         response = self.fetch("/", headers={"Accept-Language": "fr-FR; q=0.9"})
         self.assertEqual(response.headers["Content-Language"], "fr-FR")
 
+        response = self.fetch("/", headers={"Accept-Language": "fr-FR; foo=bar; q=0.9"})
+        self.assertEqual(response.headers["Content-Language"], "fr-FR")
+
+        response = self.fetch(
+            "/", headers={"Accept-Language": "fr-FR; foo=bar; q = 0.9"}
+        )
+        self.assertEqual(response.headers["Content-Language"], "fr-FR")
+
     def test_accept_language_ignore(self):
         response = self.fetch("/", headers={"Accept-Language": "fr-FR;q=0"})
         self.assertEqual(response.headers["Content-Language"], "en-US")

tornado/web.py

@@ -1292,17 +1292,21 @@ def get_browser_locale(self, default: str = "en_US") -> tornado.locale.Locale:
             locales = []
             for language in languages:
                 parts = language.strip().split(";")
-                if len(parts) > 1 and parts[1].strip().startswith("q="):
+                score = 1.0
+                for part in (
+                    x.split("=", 1)[1].strip()
+                    for x in parts[1:]
+                    if "=" in x and x.split("=", 1)[0].strip() == "q"
+                ):
                     try:
-                        score = float(parts[1].strip()[2:])
+                        score = float(part)
                         if score < 0:
                             raise ValueError()
                     except (ValueError, TypeError):
                         score = 0.0
-                else:
-                    score = 1.0
+                    break
                 if score > 0:
-                    locales.append((parts[0], score))
+                    locales.append((parts[0].strip(), score))
             if locales:
                 locales.sort(key=lambda pair: pair[1], reverse=True)
                 codes = [loc[0] for loc in locales]