Handle properties that throw KeyError

[dmfallak]

Problem:
Sometimes we can get exceptions like the following:

Traceback (most recent call last):
       <Redacted>
       ...
       File "/code/virtualenv_run/lib/python3.10/site-packages/pyramid_zipkin/tween.py", line 218, in tween
         get_binary_annotations(request, response),
       File "/code/virtualenv_run/lib/python3.10/site-packages/pyramid_zipkin/request_helper.py", line 173, in get_binary_annotations
         'network.protocol.version': request.http_version,
       File "/code/virtualenv_run/lib/python3.10/site-packages/webob/descriptors.py", line 44, in fget
         return req.environ[key]
     KeyError: 'SERVER_PROTOCOL'

Solution
Add a safety check around the property accesses.

Verification
Added a unit test to cover this case. Existing tests still pass.

[acer618]

getattr(obj, prop, default) provides a way to specify a default value if the attribute isn't found, hence avoiding the need for exception handling?

[dmfallak]

It still throws KeyError if we use getattr(obj, prop, default). I think it's because it's not that the property doesn't exist, but because this dictionary lookup fails: https://github.com/Pylons/webob/blob/5a67a56d32e3b7dd23b644b2c89736b799e50dc7/src/webob/descriptors.py#L24

[EOjeah]

@dmfallak was working on a slightly different approach with #133