Fix panic in buf curl that can happen with some map fields

[jhump]

When -v is used, the command shows the amount of unrecognized data in the response. When it counts that data, a bug in one of the case expressions allowed it to fall into the wrong case (for a map whose values are not messages). This would then result in a panic:

panic: type mismatch: cannot convert map to message

The panic actually happens whether or not the user includes a -v option, since this part of the code calculates and prints the value unconditionally. (If -v is not specified, the "print" step is effectively a no-op.)

This branch fixes that panic and adds a test. The test uses a message with fields of every possible type (including groups) to make sure that all types are correctly handled without panic. The test reproduces the panic issue, and confirms the fix.

[mattrobenolt]

🙏