The document must have only one ID

[haeungun]

If multiple IDs are declared, an exception must be thrown.
Make sure that multiple document IDs are not declared.

indexer4j/src/main/java/com/haeungun/indexer4j/core/DocumentExtractor.java

Line 51 in 319dda5

private Object extractDocumentId(T doc) throws IllegalAccessException, UndefinedDocumentIdException {