BF: isolate ImageOpener extension list

ImageOpener extension list was pointer to dictionary from Opener class,
so modifying ImageOpener modified Opener.

Author: matthew-brett

nibabel/openers.py

@@ -155,6 +155,7 @@ class ImageOpener(Opener):
     attributes, via the `register_ex_from_images`.  The class can therefore
     change state when image classes are defined.
     """
+    compress_ext_map = Opener.compress_ext_map.copy()
 
     @classmethod
     def register_ext_from_image(opener_klass, ext, func_def):

nibabel/tests/test_openers.py

@@ -57,6 +57,7 @@ def test_Opener():
         # mode is gently ignored
         fobj = Opener(obj, mode='r')
 
+
 def test_Opener_various():
     # Check we can do all sorts of files here
     message = b"Oh what a giveaway"
@@ -84,11 +85,13 @@ def test_Opener_various():
                     # Just check there is a fileno
                     assert_not_equal(fobj.fileno(), 0)
 
+
 def test_BinOpener():
     with error_warnings():
         assert_raises(DeprecationWarning,
                       BinOpener, 'test.txt', 'r')
 
+
 class TestImageOpener:
     alternate_exts = ()
     def setUp(self):
@@ -123,6 +126,9 @@ def file_opener(fileish, mode):
                 pass
             assert_true(os.path.exists('test.foo'))
 
+        # Check this doesn't add anything to parent
+        assert_false('.foo' in Opener.compress_ext_map)
+
 
 def test_file_like_wrapper():
     # Test wrapper using BytesIO (full API)