[SPARK-7991] [PySpark] Adding support for passing lists to describe.

This is a minor change.

Author: amey <amey@skytree.net>

Closes #6655 from ameyc/JIRA-7991/support-passing-list-to-describe and squashes the following commits:

e8a1dff [amey] Adding support for passing lists to describe.

python/pyspark/sql/dataframe.py

@@ -616,7 +616,19 @@ class DataFrame(object):
         |    min|  2|
         |    max|  5|
         +-------+---+
+        >>> df.describe(['age', 'name']).show()
+        +-------+---+-----+
+        |summary|age| name|
+        +-------+---+-----+
+        |  count|  2|    2|
+        |   mean|3.5| null|
+        | stddev|1.5| null|
+        |    min|  2|Alice|
+        |    max|  5|  Bob|
+        +-------+---+-----+
         """
+        if len(cols) == 1 and isinstance(cols[0], list):
+            cols = cols[0]
         jdf = self._jdf.describe(self._jseq(cols))
         return DataFrame(jdf, self.sql_ctx)