如何从pg中往hugegraph导入date类型

```
java.lang.IllegalStateException: Only accept FileSource when convert String value to Date, but got 'com.baidu.hugegraph.loader.source.jdbc.JDBCSource'
at com.google.common.base.Preconditions.checkState(Preconditions.java:172) ~[guava-11.0.2.jar:?]
at com.baidu.hugegraph.util.E.checkState(E.java:68) ~[hugegraph-common-1.6.11.jar:1.6.11.0]
at com.baidu.hugegraph.loader.util.DataTypeUtil.parseSingleValue(DataTypeUtil.java:119) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.util.DataTypeUtil.convert(DataTypeUtil.java:65) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.builder.ElementBuilder.validatePropertyValue(ElementBuilder.java:194) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.builder.ElementBuilder.addProperties(ElementBuilder.java:173) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.builder.VertexBuilder.build(VertexBuilder.java:64) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.builder.VertexBuilder.build(VertexBuilder.java:35) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.builder.ElementBuilder.next(ElementBuilder.java:110) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.HugeGraphLoader.load(HugeGraphLoader.java:186) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.HugeGraphLoader.load(HugeGraphLoader.java:161) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.HugeGraphLoader.load(HugeGraphLoader.java:132) ~[hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.HugeGraphLoader.load(HugeGraphLoader.java:89) [hugegraph-loader-0.10.1.jar:?]
at com.baidu.hugegraph.loader.HugeGraphLoader.main(HugeGraphLoader.java:66) [hugegraph-loader-0.10.1.jar:?]
2020-04-30 10:28:21 [main] [INFO ] com.baidu.hugegraph.loader.HugeGraphLoader [] - Stop loading then shutdown HugeGraphLoader
2020-04-30 10:28:21 [main] [INFO ] com.baidu.hugegraph.loader.task.TaskManager [] - Waiting for the insert tasks of vertices finish
2020-04-30 10:28:21 [main] [INFO ] com.baidu.hugegraph.loader.task.TaskManager [] - The batch-mode tasks finished
2020-04-30 10:28:21 [main] [INFO ] com.baidu.hugegraph.loader.task.TaskManager [] - The single-mode tasks finished
2020-04-30 10:28:21 [main] [DEBUG] com.baidu.hugegraph.loader.task.TaskManager [] - Ready to shutdown batch-mode tasks executor
2020-04-30 10:28:21 [main] [DEBUG] com.baidu.hugegraph.loader.task.TaskManager [] - Ready to shutdown single-mode tasks executor
2020-04-30 10:28:21 [Thread-1] [INFO ] com.baidu.hugegraph.loader.HugeGraphLoader [] - Stop loading then shutdown HugeGraphLoader
2020-04-30 10:28:21 [Thread-1] [DEBUG] com.baidu.hugegraph.loader.task.TaskManager [] - Ready to shutdown batch-mode tasks executor
2020-04-30 10:28:21 [Thread-1] [DEBUG] com.baidu.hugegraph.loader.task.TaskManager [] - Ready to shutdown single-mode tasks executor
```

Start by reproducing the PostgreSQL-to-HugeGraph load described in the issue, then inspect DataTypeUtil.parseSingleValue() and DataTypeUtil.convert(), followed by ElementBuilder.validatePropertyValue(). The work is done when the reported date value can be imported through JDBCSource without the shown exception; no test file is named in the issue.

Written by the indexing model from the issue text.