[SPARK-11371] Make "mean" an alias for "avg" operator

From Reynold in the thread 'Exception when using some aggregate operators' (http://search-hadoop.com/m/q3RTt0xFr22nXB4/):

I don't think these are bugs. The SQL standard for average is "avg", not "mean". Similarly, a distinct count is supposed to be written as "count(distinct col)", not "countDistinct(col)".
We can, however, make "mean" an alias for "avg" to improve compatibility between DataFrame and SQL.

Author: tedyu <yuzhihong@gmail.com>

Closes #9332 from ted-yu/master.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -185,6 +185,7 @@ object FunctionRegistry {
     expression[Last]("last"),
     expression[Last]("last_value"),
     expression[Max]("max"),
+    expression[Average]("mean"),
     expression[Min]("min"),
     expression[Stddev]("stddev"),
     expression[StddevPop]("stddev_pop"),

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/AggregationQuerySuite.scala

@@ -298,6 +298,15 @@ abstract class AggregationQuerySuite extends QueryTest with SQLTestUtils with Te
         """.stripMargin),
       Row(1, 20.0) :: Row(2, -0.5) :: Row(3, null) :: Row(null, 10.0) :: Nil)
 
+    checkAnswer(
+      sqlContext.sql(
+        """
+          |SELECT key, mean(value)
+          |FROM agg1
+          |GROUP BY key
+        """.stripMargin),
+      Row(1, 20.0) :: Row(2, -0.5) :: Row(3, null) :: Row(null, 10.0) :: Nil)
+
     checkAnswer(
       sqlContext.sql(
         """