Fix qstring.nest returning incorrect value

Fix a bug in `qstring.nest` where it returned an incorrect value when
there were more than two query parameters with the same name.

Before:

    >>> qtstring.nest([('foo', '1'), ('foo', '2'), ('foo', '3')])
    {'foo': [['1', '2'], '3']}

After:

    >>> qtstring.nest([('foo', '1'), ('foo', '2'), ('foo', '3')])
    {'foo': ['1', '2', '3']}

CHANGES.rst

@@ -9,6 +9,18 @@ Changelog
 - Changed ``qstring.nest`` to use ``dict`` in the returned nested object instead
   of ``OrderedDict``. ``dict`` retains insertion order since Python 3.7, so
   ``OrderedDict`` usage was redundant here.
+- Fixed a bug in ``qstring.nest`` where it returned an incorrect value when
+  there were more than two query parameters with the same name.
+
+  Before::
+
+      >>> qtstring.nest([('foo', '1'), ('foo', '2'), ('foo', '3')])
+      {'foo': [['1', '2'], '3']}
+
+  After::
+
+      >>> qtstring.nest([('foo', '1'), ('foo', '2'), ('foo', '3')])
+      {'foo': ['1', '2', '3']}
 
 0.2.1 (March 24, 2017)
 ^^^^^^^^^^^^^^^^^^^^^^

qstring/nest.py

@@ -38,7 +38,11 @@ def _convert_params_list_to_dict(params_list):
     params_dict = {}
     for key, value in params_list:
         if key in params_dict:
-            params_dict[key] = [params_dict[key], value]
+            params_dict[key] = (
+                params_dict[key] + [value]
+                if isinstance(params_dict[key], list)
+                else [params_dict[key], value]
+            )
         else:
             params_dict[key] = value
     return params_dict

tests/test_nest.py

@@ -13,7 +13,7 @@
         ([('', 'bar')], {'': 'bar'}),
         ([('foo', 'bar')], {'foo': 'bar'}),
         ([('foo', ''), ('bar', '')], {'foo': '', 'bar': ''}),
-        ([('foo', '1'), ('foo', '2')], {'foo': ['1', '2']}),
+        ([('foo', '1'), ('foo', '2'), ('foo', '3')], {'foo': ['1', '2', '3']}),
         ([('foo', '1'), ('bar', '2')], {'foo': '1', 'bar': '2'}),
         ([('x[y]', '1')], {'x': {'y': '1'}}),
         ([('x[y][z]', '1')], {'x': {'y': {'z': '1'}}}),