Skip comparison of a field if it's unset in both messages

[jarodmeng]

This fixes #56.

library(RProtoBuf)

# Create an example proto file with recursive field
proto.file <- tempfile(fileext = ".proto")
writeLines(
  "
  syntax = 'proto2';

  package example;

  message Recursion {
      optional int32 id = 1;
      optional Recursion value = 2;
  }
  ", proto.file
)
readProtoFiles(proto.file)

# Create two identical proto messages with recursive field unset
p <- new(example.Recursion, id = 1)
p2 <- new(example.Recursion, id = 1)
# Create a different proto message
p3 <- new(example.Recursion, id = 2)

# identical() and all.equal() now work even for recursive fields
identical(p, p2)
#> [1] TRUE
all.equal(p, p2)
#> [1] TRUE

identical(p, p3)
#> [1] FALSE
all.equal(p, p3)
#> [1] FALSE

^{Created on 2018-11-30 by the reprex package (v0.2.1)}