core.http_request: make get_input_headers() lower case header names

--HG--
extra : transplant_source : V%1F%3F%E8%12%A6C%7DS%26%D8%92%13%1FL%02%26%EB%8D%C5

gevent/evhttp.pxi

@@ -220,7 +220,9 @@ cdef class http_request:
         while p:
             key = p.key
             value = p.value
-            result.append((key if key else None, value if value else None))
+            if key == NULL or value == NULL:
+                break
+            result.append((key.lower(), value))
             p = TAILQ_GET_NEXT(p)
         return result