salt_cherrypy with Python3 and complex CORS OPTION requests return HTTP 500

[niflostancu]

Description of Issue

I am trying to use salt_cherrypy from a browser application (uses complex CORS), but the OPTION requests return 500 error and the following exception from CherryPy:
ValueError: Page handlers MUST return bytes. Use tools.encode if you wish to return unicode at salt/netapi/rest_cherrypy/app.py:824 (see below for full traceback).

A related issue on CherryPy might be cherrypy/cherrypy#1123

Setup

Relevant master config (SSL from letsencrypt):

rest_cherrypy:
  port: 8000
  ssl_crt: /etc/letsencrypt/live/***/fullchain.pem
  ssl_key: /etc/letsencrypt/live/***/privkey.pem

Steps to Reproduce Issue

Running (JavaScript, browser fetch API):

fetch("https://saltstack.domain:8000/login", 
     method: "POST",
     headers: {"Accept": "application/json", "Content-Type": "application/json"},
     body: JSON.stringify({username: "***", password: "***", eauth: "file"},     
)

Results in HTTP 500 on the OPTION request on the browser + the following log from salt-api:

[ERROR   ] [22/Oct/2019:08:57:17]  Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cherrypy/_cprequest.py", line 104, in run
    hook()
  File "/usr/lib/python3/dist-packages/cherrypy/_cprequest.py", line 63, in __call__
    return self.callback(**self.kwargs)
  File "/usr/lib/python3/dist-packages/salt/netapi/rest_cherrypy/app.py", line 824, in cors_tool
    cherrypy.response.body = ''
  File "/usr/lib/python3/dist-packages/cherrypy/__init__.py", line 231, in __setattr__
    setattr(child, name, value)
  File "/usr/lib/python3/dist-packages/cherrypy/_cprequest.py", line 816, in __set__
    raise ValueError(self.unicode_err)
ValueError: Page handlers MUST return bytes. Use tools.encode if you wish to return unicode.

Workaround

I manually modified salt/netapi/rest_cherrypy/app.py:824 into:

        cherrypy.response.body = bytes('', 'UTF-8')

And the error was fixed.
Can this be fixed on master or should I come with a pull request (it's a pretty trivial change)?

Versions Report

Salt Version:
           Salt: 2019.2.1

Dependency Versions:
           cffi: Not Installed
       cherrypy: 3.5.0
       dateutil: 2.4.2
      docker-py: Not Installed
          gitdb: 0.6.4
      gitpython: 1.0.1
          ioflo: Not Installed
         Jinja2: 2.8
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.6
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.5.2 (default, Oct  8 2019, 13:06:37)
   python-gnupg: 0.3.8
         PyYAML: 3.11
          PyZMQ: 15.2.0
           RAET: Not Installed
          smmap: 0.9.0
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4

System Versions:
           dist: Ubuntu 16.04 xenial
         locale: UTF-8
        machine: x86_64
        release: 4.15.0-65-generic
         system: Linux
        version: Ubuntu 16.04 xenial

Thanks.

[xeacott]

Go ahead and generate a PR and target master. We'll continue the conversation in there. I also do not know if there are tests for this module but a PR would require a partner test with the change, regardless of how trivial the change is but please submit one!