`Marshal()` includes `"default="` tags in `EnvSet` output

I'm interested in using env.Marshal() to facilitate logging the environment variables that are set in build configuration for several Paketo Buildpacks.

We'd like to use the default= tag on some of our configuration struct fields. But when I iterate over the output of env.Marshal(&myStruct) to log the values of the environment variables, I see that the tags default=* are included as keys in the EnvSet.

See this Go playground for an example of what I mean.

@jimmykodes can you help with this?