Version 0.5.1 Error: DataSource specified is not org.embulk.config.DataSourceImpl

Hello, it seems that version 0.5.1 introduces some kind of incompatibilty with embulk-config-utils or embulk-api,
I've downgraded to 0.5.0 and it worked just fine. My config is very basic, something like:

```
in:
type: s3
bucket: some-bucket
path_prefix: ""
access_key_id: xxx
secret_access_key: xxxx
parser:
type: none
decoders:
- type: gzip
out:
type: stdout
```

The logs were the following:

```
Caused by: java.lang.ClassCastException: DataSource specified is not org.embulk.config.DataSourceImpl.
at org.embulk.util.config.Compat.callGetObjectNodeAndRebuildIfAvailable(Compat.java:136)
at org.embulk.util.config.Compat.rebuildObjectNode(Compat.java:94)
at org.embulk.util.config.TaskMapper.map(TaskMapper.java:73)
at org.embulk.input.s3.AbstractS3FileInputPlugin.resume(AbstractS3FileInputPlugin.java:162)
at org.embulk.input.s3.AbstractS3FileInputPlugin.transaction(AbstractS3FileInputPlugin.java:153)
at org.embulk.spi.FileInputRunner.transaction(FileInputRunner.java:62)
at org.embulk.exec.BulkLoader.doRun(BulkLoader.java:507)
... 11 more

```

This message comes from here:

https://github.com/embulk/embulk-util-config/blob/500599620de1dfcfe31f876323a4fb893d03fb27/src/main/java/org/embulk/util/config/Compat.java#L136

Not sure the cause, maybe the getClass from the s3 pluginf file is nor returning the data source impl?

thank you!