Generalised the name of the expected InputSplit class in HadoopDataSource.

In order to get the name of an ```InputSplit``` class  ```getCanonicalName()``` has been used so far in ```HadoopInputSplitWrapper```. This is ok for most cases, except for the case when the ```InputSplit``` is a nested class inside its ```InputFormat``` class. 

Specifically, here is an exact case:  https://github.com/Parquet/parquet-mr/blob/master/parquet-hadoop/src/main/java/parquet/hadoop/mapred/DeprecatedParquetInputFormat.java

This fixes https://github.com/stratosphere/stratosphere-experimental/issues/3