zero numerics on optional fields are not encoded

[dysonreturns]

Given a message with an optional int

syntax = "proto3";

message Takeout {
    // nil (unspecified), 0 (no cutlery), 1 (1 set of cutlery), etc.
    optional uint32 cutlery_needed = 1;
    // Other fields...
}

The official Google proto library encodes these values as such:

Takeout.new(cutlery_needed:0).to_proto
# => "\b\x00"
Takeout.new(cutlery_needed:nil).to_proto
# => ""

When protoboeuf encodes this message, then it should match the output above:

Takeout.new(cutlery_needed:0).to_proto
=> ""
Takeout.new(cutlery_needed:nil).to_proto
=> ""

Notice how the 0 (zero) value is considered being unset and isn't transmitted "\b\x00".
This currentl breaks cross-language compatibility between these two libs and therefore also backwards compat with proto2.

Not sure how much it's going to cost, but we need to check the @_bitmask instead of val != 0 in these cases for whether something is set or not.