produces invalid json as values are not quoted

[keis]

Running the example from the README with my dependencies define like so

[dependencies]
json_env_logger = { version = "0.1", features = ["iso-timestamps"] }
kv-log-macro = "1.0"
log = ">=0.4.13, <0.4.14"

Limiting the log version as per #6

The output I'm seeing looks like
{"level":"INFO","ts":"2021-03-07T17:21:26.043Z","msg":"👋","size":medium,"age":42}

Notice how the string "medium" is not quoted resulting in invalid json.