python: allow passing single strings instead of lists in more places

Several of our APIs take lists of column names, but it's easy to
accidentally just pass a string. If we naively iterate over the string,
we end up interpreting "foo" as ["f", "o", "o"] which is obviously
incorrect.

This patch takes the approach of detecting the string argument and
converting to a singleton list.

An alternative would have been to raise TypeError, but it seems like we
do this conversion in other places, so I decided to be consistent.

Change-Id: I1a81dea5356b66b8860d22f9ee2935072fd4cd6c
Reviewed-on: http://gerrit.cloudera.org:8080/5019
Tested-by: Kudu Jenkins
Reviewed-by: Jordan Birdsell <[email protected]>

python/kudu/client.pyx

@@ -1023,6 +1023,8 @@ class Partitioning(object):
         -------
         self: this object
         """
+        if isinstance(column_names, str):
+            column_names = [column_names]
         self._range_partition_cols = column_names
         return self
 
@@ -1464,6 +1466,8 @@ cdef class Scanner:
         -------
         self : Scanner
         """
+        if isinstance(names, str):
+            names = [names]
         cdef vector[string] v_names
         for name in names:
             v_names.push_back(tobytes(name))
@@ -1930,6 +1934,8 @@ cdef class ScanTokenBuilder:
         -------
         self : ScanTokenBuilder
         """
+        if isinstance(names, str):
+            names = [names]
         cdef vector[string] v_names
         for name in names:
             v_names.push_back(tobytes(name))