uasyncio: benchmark: Modern browsers produce requests more than 256 bytes.

So, not all of the request was read, and a browser would report than
connection was reset.

uasyncio/benchmark/test_http_server_light.py

@@ -5,7 +5,7 @@ import uasyncio as asyncio
 def serve(reader, writer):
     #print(reader, writer)
     #print("================")
-    yield from reader.read(256)
+    yield from reader.read(512)
     yield from writer.awrite("HTTP/1.0 200 OK\r\n\r\nHello.\r\n")
     yield from writer.aclose()
     #print("Finished processing request")

uasyncio/benchmark/test_http_server_medium.py

@@ -6,7 +6,7 @@ resp = "HTTP/1.0 200 OK\r\n\r\n" + "Hello.\r\n" * 1500
 def serve(reader, writer):
     #print(reader, writer)
     #print("================")
-    yield from reader.read(256)
+    yield from reader.read(512)
     yield from writer.awrite(resp)
     yield from writer.aclose()
     #print("Finished processing request")