Fix bug in http server for serving files (#3995)

Author: paulsemel

src/clusterfuzz/_internal/bot/webserver/http_server.py

@@ -101,7 +101,10 @@ def do_GET(self):  # pylint: disable=invalid-name
       return
 
     try:
-      with open(absolute_path) as file_handle:
+      # It is necessary to open the file as binary because `self.wfile` is now
+      # an `io.BufferedIOBase` since python3.6, and thus expects binary input
+      # in the `self.wfile.write` call below.
+      with open(absolute_path, 'rb') as file_handle:
         data = file_handle.read()
     except OSError:
       self.send_response(403)

src/clusterfuzz/_internal/tests/core/bot/webserver/http_server_test.py

@@ -32,7 +32,9 @@ def __init__(self):
       self.contents = ''
 
     def write(self, data):
-      self.contents += data
+      # Since the fake files are being created with simple contents, we can
+      # just decode that as 'utf-8'.
+      self.contents += data.decode("utf-8")
 
   def __init__(self, path):  # pylint: disable=super-init-not-called
     self.response_code = 0