[SPARK-17991][SQL] Enable metastore partition pruning by default.

## What changes were proposed in this pull request?

This should apply to non-converted metastore relations. WIP to see if this causes any test failures.

## How was this patch tested?

Existing tests.

Author: Eric Liang <ekl@databricks.com>

Closes #15475 from ericl/try-enabling-pruning.

sql/core/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -267,7 +267,7 @@ object SQLConf {
       .doc("When true, some predicates will be pushed down into the Hive metastore so that " +
            "unmatching partitions can be eliminated earlier.")
       .booleanConf
-      .createWithDefault(false)
+      .createWithDefault(true)
 
   val HIVE_FILESOURCE_PARTITION_PRUNING =
     SQLConfigBuilder("spark.sql.hive.filesourcePartitionPruning")