Rename TRAILERS to TRAILER as it defined in RFC2616

Author: kxepal

aiohttp/hdrs.py

@@ -49,7 +49,7 @@
 SERVER = upstr('SERVER')
 SET_COOKIE = upstr('SET-COOKIE')
 TE = upstr('TE')
-TRAILERS = upstr('TRAILERS')
+TRAILER = upstr('TRAILER')
 TRANSFER_ENCODING = upstr('TRANSFER-ENCODING')
 UPGRADE = upstr('UPGRADE')
 URI = upstr('URI')

aiohttp/protocol.py

@@ -801,7 +801,7 @@ class Response(HttpMessage):
         hdrs.PROXY_AUTHENTICATE,
         hdrs.PROXY_AUTHORIZATION,
         hdrs.TE,
-        hdrs.TRAILERS,
+        hdrs.TRAILER,
         hdrs.TRANSFER_ENCODING,
         hdrs.UPGRADE,
     }