allow for lowercased metadata fields

some mlses break with the spec here:

```
c.capabilities.keys.sort
=> ["broker",
 "getmetadata",
 "getobject",
 "login",
 "logout",
 "membername",
 "metadatatimestamp",
 "metadataversion",
 "minmetadatatimestamp",
 "search",
 "timeoutseconds",
 "user"]
```

This should work around mlses that have incorrect fields like this.