FIX: standardize case of keys for properties from MLSD command

According to RFC 3659 https://tools.ietf.org/html/rfc3659#section-7.2 the MLSD responses for the properties of a file are not all lower case. They are camel case "Size", "Type", "Modified", etc. The previous code would throw an error unless the keys were lowercase. So convert and lookup keys with lowercase only.