Reusing a reserved property name such as `msg` leads to an incorrect json output.

Possible resolutions:

1) Outlaw the use of reserved properties (i.e. produce a compile-time error when they are used)
2) Choose new names for the reserved properties that are less likely to collide with user code (e.g. `$msg`, `$lvl`, etc)
3) Nest all user properties in a dedicated sub-field of the record.

The most likely initial resolution will be option 1).