enable CSV Reader to ignore invalid lines like an empty line at the end and comments - FLINK-1208

Corresponding to issue FLINK-1208:
enable CSV Reader to ignore invalid lines like an empty line at the end, multiple header lines or comments without throwing an exception
in DataSourceTask the previous record is used to get the next record:

```java
while (!this.taskCanceled && !format.reachedEnd()) {
	// build next pair and ship pair if it is valid
	if ((record = format.nextRecord(record)) != null) {
		output.collect(record);
	}
}
```
The problem is: when there was an invalid record, the function nextRecord has returned null and  the null record will stop the loop to get more records ...