Fix #1035 unclosed file warning in GiftiImage

- Change as proposed by @effigies

Author: HippocampusGirl

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, mmap=True):