Fix null parameters

Closes #55

pyhive/common.py

@@ -238,7 +238,9 @@ class ParamEscaper(object):
         return "'{}'".format(item.replace("'", "''"))
 
     def escape_item(self, item):
-        if isinstance(item, (int, float)):
+        if item is None:
+            return 'NULL'
+        elif isinstance(item, (int, float)):
             return self.escape_number(item)
         elif isinstance(item, basestring):
             return self.escape_string(item)

pyhive/tests/dbapi_test_case.py

@@ -45,6 +45,11 @@ class DBAPITestCase(with_metaclass(abc.ABCMeta, object)):
         cursor.execute('SELECT a FROM many_rows ORDER BY a')
         self.assertEqual(cursor.fetchall(), [(i,) for i in range(10000)])
 
+    @with_cursor
+    def test_null_param(self, cursor):
+        cursor.execute('SELECT %s FROM one_row', (None,))
+        self.assertEqual(cursor.fetchall(), [(None,)])
+
     @with_cursor
     def test_iterator(self, cursor):
         cursor.execute('SELECT * FROM one_row')