[Rust] Return None for optional primite field if its version is less than actingVersion.

[marciorasf]

There's a treatment in optional fields to return None for fields which the version is less than the message actingVersion. This kind of treatment existed for all types of fields, except for optional primitives.

I implemented the treatment and added a test for this case: !1067

[vyazelenko]

PR #1067 merged.