fix and tests for the issue related with the trailing slash using 'nbextension install' (see #2069)

notebook/nbextensions.py

@@ -165,7 +165,7 @@ def install_nbextension(path, overwrite=False, symlink=False,
         full_dest = None
     else:
         if not destination:
-            destination = basename(path)
+            destination = basename(normpath(path))
         destination = cast_unicode_py2(destination)
         full_dest = normpath(pjoin(nbext, destination))
         if overwrite and os.path.lexists(full_dest):

notebook/tests/test_nbextensions.py

@@ -178,6 +178,11 @@ class TestInstallNBExtension(TestCase):
         install_nbextension(pjoin(self.src, d))
         self.assert_installed(self.files[-1])
     
+    def test_single_dir_trailing_slash(self):
+        d = [u'∂ir/', u'∂ir\\']
+        for dest in d:
+            install_nbextension(pjoin(self.src, dest))
+            self.assert_installed(self.files[-1])    
 
     def test_destination_file(self):
         file = self.files[0]