[SPARK-1938] [SQL] ApproxCountDistinctMergeFunction should return Int value.

`ApproxCountDistinctMergeFunction` should return `Int` value because the `dataType` of `ApproxCountDistinct` is `IntegerType`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #893 from ueshin/issues/SPARK-1938 and squashes the following commits:

3970e88 [Takuya UESHIN] Remove a superfluous line.
5ad7ec1 [Takuya UESHIN] Make dataType for each of CountDistinct, ApproxCountDistinctMerge and ApproxCountDistinct LongType.
cbe7c71 [Takuya UESHIN] Revert a change.
fc3ac0f [Takuya UESHIN] Fix evaluated value type of ApproxCountDistinctMergeFunction to Int.
Fix doc about NetworkWordCount/JavaNetworkWordCount usage of spark streaming

Usage: NetworkWordCount <master> <hostname> <port>
-->
Usage: NetworkWordCount <hostname> <port>

Usage: JavaNetworkWordCount <master> <hostname> <port>
-->
Usage: JavaNetworkWordCount <hostname> <port>

Author: jmu <jmujmu@gmail.com>

Closes #826 from jmu/master and squashes the following commits:

9fb7980 [jmu] Merge branch 'master' of https://github.com/jmu/spark
b9a6b02 [jmu] Fix doc for NetworkWordCount/JavaNetworkWordCount Usage: NetworkWordCount <master> <hostname> <port> --> Usage: NetworkWordCount <hostname> <port>