IA: generate sourceResource.title correctly (fixes #7708)

Given IA metadata like the following:
```json
{"title": ["Foo", "bar"], "volume": "1880"}
```
IAMapper should generate sourceResource.title as such:
```json
{"sourceResource": {"title": ["Foo, 1880", "bar, 1880"]}}
```

See https://issues.dp.la/issues/7708
Since either extend() or append() can be used on a list, could there be an explanation of why these are used in different circumstances?  If the argument could be something other than a list, then it might be more clear if it could be named differently.