Support Map Fields With Boolean Keys

The protobuf specification doesn't disallow booleans from appearing as keys in maps. I'm not entirely sure why one would want this, but currently pbjson produces code that compiles for such fields, but errors during encode with

```
Error("key must be a string", line: 0, column: 0)
```

This should be fixed

Locate the generated map-field encode path and reproduce the failure with a protobuf map whose key is boolean. Compare it with the supported map-key cases, then add coverage for boolean keys and verify that encoding completes without the reported key-string error.

Written by the indexing model from the issue text.