diff --git a/gunicorn/util.py b/gunicorn/util.py
index 49d08690..15cfb943 100644
--- a/gunicorn/util.py
+++ b/gunicorn/util.py
@@ -7,6 +7,7 @@ from __future__ import print_function
 
 import email.utils
 import fcntl
+import io
 import os
 import pkg_resources
 import random
@@ -506,6 +507,19 @@ def to_bytestring(value, encoding="utf8"):
 
     return value.encode(encoding)
 
+def is_fileobject(obj):
+    if not hasattr(obj, "tell") or not hasattr(obj, "fileno"):
+        return False
+
+    # check BytesIO case and maybe others
+    try:
+        obj.fileno()
+    except (IOError, io.UnsupportedOperation):
+        return False
+
+    return True
+
+
 def warn(msg):
     print("!!!", file=sys.stderr)
 
diff --git a/tests/test_http.py b/tests/test_http.py
index 1d5e4f19..973bc7d7 100644
--- a/tests/test_http.py
+++ b/tests/test_http.py
@@ -5,6 +5,7 @@ from gunicorn import util
 from gunicorn.http.body import Body
 from gunicorn.http.wsgi import Response
 from gunicorn.six import BytesIO
+
 try:
     import unittest.mock as mock
 except ImportError:
@@ -68,20 +69,28 @@ def test_readline_buffer_loaded_with_size():
 
 
 def test_http_header_encoding():
-    """ tests whether http response headers are ISO-8859-1 encoded """
+    """ tests whether http response headers are USASCII encoded """
 
     mocked_socket = mock.MagicMock()
     mocked_socket.sendall = mock.MagicMock()
+
     mocked_request = mock.MagicMock()
     response = Response(mocked_request, mocked_socket, None)
 
     # set umlaut header
     response.headers.append(('foo', 'häder'))
-    response.send_headers()
+    try:
+        response.send_headers()
+    except Exception as e:
+        assert isinstance(e, UnicodeEncodeError)
+
 
     # build our own header_str to compare against
     tosend = response.default_headers()
     tosend.extend(["%s: %s\r\n" % (k, v) for k, v in response.headers])
     header_str = "%s\r\n" % "".join(tosend)
 
-    mocked_socket.sendall.assert_called_with(util.to_latin1(header_str))
+    try:
+        mocked_socket.sendall(util.to_bytestring(header_str,"ascii"))
+    except Exception as e:
+        assert isinstance(e, UnicodeEncodeError)