as_document and load have dissimilar results for CommaSeparated(Int())

```python
>>> load('1, 1', CommaSeparated(Int()))
YAML([1, 1])
```
```python
>>> load('1,1', CommaSeparated(Int()))
YAML([1, 1])
```
```python
>>> as_document('1,1', CommaSeparated(Int()))
YAML([1, 1])
```
```python
>>> as_document('1, 1', CommaSeparated(Int()))
strictyaml.exceptions.YAMLSerializationError: ' 1' not an integer.
```