BooleanFilterSpec fails to deserialize

FilterSpec serialization/deserialization relies on being able to parse the result of FilterSpec.toString() via the static fromString(...) method.

The following toString() -> fromString(...) operation sequences fail with an IllegalArgumentException: "Unexpected character at 0"

FilterSpec.fromString(FilterSpec.all().toString());
FilterSpec.fromString(FilterSpec.none().toString());
// These fail with "Unexpected character at 2"
FilterSpec.fromString(FilterSpec.not(FilterSpec.all()).toString());
FilterSpec.fromString(FilterSpec.not(FilterSpec.none()).toString());

Consequently, attempts to deserialize the serialized representations of FilterSpec.all() or FilterSpec.none() will also fail.