optimise response connection header check

Get the status code from the response once so we can use it to check the need
for the connection header later without parsing the string each time we need it.

gunicorn/http/wsgi.py

@@ -194,8 +194,7 @@ class Response(object):
             return True
         if self.response_length is not None or self.chunked:
             return False
-        status = self.status.split()[0] if self.status else ''
-        if status.startswith('1') or status in ('204', '304'):
+        if self.status_code < 200 or self.status_code in (204, 304):
             return False
         return True
 
@@ -210,6 +209,15 @@ class Response(object):
             raise AssertionError("Response headers already set!")
 
         self.status = status
+
+        # get the status code from the response here so we can use it to check
+        # the need for the connection header later without parsing the string
+        # each time.
+        try:
+            self.status_code = int(self.status.split()[0])
+        except ValueError:
+            self.status_code = None
+
         self.process_headers(headers)
         self.chunked = self.is_chunked()
         return self.write
@@ -243,7 +251,7 @@ class Response(object):
             return False
         elif self.req.version <= (1, 0):
             return False
-        elif self.status.startswith("304") or self.status.startswith("204"):
+        elif self.status_code in (204, 304):
             # Do not use chunked responses when the response is guaranteed to
             # not have a response body.
             return False