fix: Message.to_pydict and .from_pydict

[RF-Tar-Railt]

Summary

resolve #610

Just one line different:

• to_dict:

try:
    value = getattr(self, field_name)
except AttributeError:
    value = self._get_field_default(field_name)
cased_name = casing(field_name).rstrip("_")  # type: ignore

• to_pydict:

value = getattr(self, field_name)
cased_name = casing(field_name).rstrip("_")  # type: ignore

from_pydict is the same cause.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Gobot1234]

What is this fixing? Please can you also provide tests so that this doesn't happen in the future

[RF-Tar-Railt]

What is this fixing? Please can you also provide tests so that this doesn't happen in the future

updated

[straygar]

Could a reviewer please take a look at this? It makes protobufs containing oneof unusable with v2.0.0b7.

[straygar]

@cetanu Thx for reviewing! When do branches get merged?