The PUT views should return an empty bytes string.

If they don't, on Python 3 we run afoul of the zope.publisher rule
that says that unicode response bodies need to have a content-type.
But since the put view never sets a content type (due to being a
redirect), we get a TypeError instead:

```
  File "/py35/zope/app/wsgi/testlayer.py", line 210, in http
    response = request.get_response(wsgi_app)
  File "/py35/webob/request.py", line 1316, in send
    application, catch_exc_info=False)
  File "/py35/webob/request.py", line 1283, in call_application
    output.extend(app_iter)
  File "/py35/zope/app/wsgi/testlayer.py", line 94, in __call__
    for entry in self.wsgi_stack(environ, application_start_response):
  File "/py35/zope/app/wsgi/testlayer.py", line 68, in __call__
    for entry in self.wsgi_stack(environ, start_response):
  File "/py35/zope/app/wsgi/__init__.py", line 65, in __call__
    request = publish(request, handle_errors=handle_errors)
  File "/py35/zope/publisher/publish.py", line 148, in publish
    response.setResult(result)
  File "/py35/zope/publisher/http.py", line 796, in setResult
    r, headers = self._implicitResult(r)
  File "/py35/zope/publisher/http.py", line 821, in _implicitResult
    if not unicode_mimetypes_re.match(ct):
TypeError: expected string or bytes-like object
```

Here, `ct` is the content-type header, and its None. Returning bytes
bypasses the issue.

This was discovered testing with zope.app.file.

CHANGES.txt

@@ -2,10 +2,12 @@
 CHANGES
 =======
 
-4.1.0 (unreleased)
+4.0.1 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- The PUT views return an empty byte string instead of an empty native
+  string. On Python 3, the unicode native string leads to a TypeError
+  in zope.publisher.http.
 
 
 4.0.0 (2016-08-08)

src/zope/app/http/put.py

@@ -88,7 +88,7 @@ def PUT(self):
         request.response.setStatus(201)
         request.response.setHeader(
             'Location', zope.traversing.browser.absoluteURL(newfile, request))
-        return ''
+        return b''
 
 
 class FilePUT(object):
@@ -111,4 +111,4 @@ def PUT(self):
         length = int(self.request.get('CONTENT_LENGTH', -1))
         adapter.write(body.read(length))
 
-        return ''
+        return b''

src/zope/app/http/tests/test_put.py

@@ -106,7 +106,7 @@ def test_bad_content_header(self):
         null = zope.app.http.put.NullResource(container, 'spam')
         put = zope.app.http.put.NullPUT(null, request)
         self.assertEqual(getattr(container, 'spam', None), None)
-        self.assertEqual(put.PUT(), '')
+        self.assertEqual(put.PUT(), b'')
         request.response.setResult('')
 
         # Check HTTP Response
@@ -136,7 +136,7 @@ def test(self):
                                'CONTENT_LENGTH': str(len(content)),
                                })
         put = zope.app.http.put.FilePUT(file, request)
-        self.assertEqual(put.PUT(), '')
+        self.assertEqual(put.PUT(), b'')
         request.response.setResult('')
         self.assertEqual(file.data, content)
 
@@ -153,7 +153,7 @@ def test_bad_content_header(self):
                                'HTTP_CONTENT_FOO': 'Bar',
                                })
         put = zope.app.http.put.FilePUT(file, request)
-        self.assertEqual(put.PUT(), '')
+        self.assertEqual(put.PUT(), b'')
         request.response.setResult('')
         self.assertEqual(file.data, content)