MapFileOutputFormat and SequenceFileOutputFormat use incorrect key/value classes in map/reduce tasks

Let's assume a job uses different key/value class for the output of map tasks and for the final output of reduce tasks.

When executing map tasks classes returned from JobConf.getMapOutputKeyClass() / getMapOutputValueClass() should be used, and when executing reduce tasks classes returned from JobConf.gtOutputKeyClass() / getOutputValueClass() should be used.

Currently both map and reduce tasks will use getMapOutputKeyClass/getMapOutputValueClass when using MapFileOutputFormat, or they will always use getOutputKeyClassgetOutputValueClass when using SequenceFileOutputFormat. This causes exceptions, because Mapper / Reducer implementations will output different key/value classes than expected.