Fix #1035 unclosed file warning in GiftiImage

- Change as proposed by @effigies

Test for `ResourceWarning` in `GiftiImage`

Backport of gh-1038

nibabel/gifti/gifti.py

@@ -877,8 +877,8 @@ def to_file_map(self, file_map=None):
         """
         if file_map is None:
             file_map = self.file_map
-        f = file_map['image'].get_prepare_fileobj('wb')
-        f.write(self.to_xml())
+        with file_map['image'].get_prepare_fileobj('wb') as f:
+            f.write(self.to_xml())
 
     @classmethod
     def from_file_map(klass, file_map, buffer_size=35000000):

nibabel/gifti/tests/test_gifti.py

@@ -6,6 +6,8 @@
 
 import numpy as np
 
+from nibabel.tmpdirs import InTemporaryDirectory
+
 from ... import load
 from .. import (GiftiImage, GiftiDataArray, GiftiLabel,
                 GiftiLabelTable, GiftiMetaData, GiftiNVPairs,
@@ -40,7 +42,7 @@ def test_agg_data():
 
     assert_array_equal(surf_gii_img.agg_data('pointset'), point_data)
     assert_array_equal(surf_gii_img.agg_data('triangle'), triangle_data)
-    assert_array_equal(func_gii_img.agg_data('time series'), func_data) 
+    assert_array_equal(func_gii_img.agg_data('time series'), func_data)
     assert_array_equal(shape_gii_img.agg_data('shape'), shape_data)
 
     assert surf_gii_img.agg_data('time series') == ()
@@ -443,3 +445,11 @@ def test_darray_dtype_coercion_failures():
         da_copy = gii_copy.darrays[0]
         assert np.dtype(da_copy.data.dtype) == np.dtype(darray_dtype)
         assert_array_equal(da_copy.data, da.data)
+
+
+def test_gifti_file_close():
+    gii = load(test_data('gifti', 'ascii.gii'))
+    with pytest.WarningsRecorder() as record:
+        with InTemporaryDirectory():
+            gii.to_filename('test.gii')
+    assert not any(isinstance(r.message, ResourceWarning) for r in record.list)