建议优化MongoDBReader代码逻辑MongoDBReader 165-177行

目前代码段:
``` if(KeyConstant.isArrayType(column.getString(KeyConstant.COLUMN_TYPE))) {
String splitter = column.getString(KeyConstant.COLUMN_SPLITTER);
if(Strings.isNullOrEmpty(splitter)) {
throw DataXException.asDataXException(MongoDBReaderErrorCode.ILLEGAL_VALUE,
MongoDBReaderErrorCode.ILLEGAL_VALUE.getDescription());
} else {
ArrayList array = (ArrayList)tempCol;
String tempArrayStr = Joiner.on(splitter).join(array);
record.addColumn(new StringColumn(tempArrayStr));
}
} else {
record.addColumn(new StringColumn(tempCol.toString()));
} ```
其中new StringColumn(tempArrayStr) ，new StringColumn(tempCol.toString()) 入mysql库后，打乱了原有json形式，直接Document的 toString
``` @Override
public String toString() {
return "Document{"
+ documentAsMap
+ '}';
}```
json格式打乱了。不如换成:
`if(KeyConstant.isArrayType(column.getString(KeyConstant.COLUMN_TYPE))) {
String splitter = column.getString(KeyConstant.COLUMN_SPLITTER);

if(Strings.isNullOrEmpty(splitter)) {
throw DataXException.asDataXException(MongoDBReaderErrorCode.ILLEGAL_VALUE,
MongoDBReaderErrorCode.ILLEGAL_VALUE.getDescription());
} else {
ArrayList array = (ArrayList)tempCol;
record.addColumn(new StringColumn(JSON.toJSONString(array)));
}
} else {
if(tempCol instanceof Document){
record.addColumn(new StringColumn(JSON.toJSONString(tempCol)));
}else{
record.addColumn(new StringColumn(tempCol.toString()));
}

}`

Start at MongoDBReader lines 165-177 and inspect how array values and Document instances are converted into StringColumn values. Compare the resulting values with the original JSON structure and verify that MongoDB data written to MySQL preserves that structure; no test file is named in the issue.

Written by the indexing model from the issue text.