Add encoding declaration to test for notebook -> .py export.

IPython/nbformat/v2/tests/nbexamples.py

@@ -81,7 +81,8 b' nb0 = new_notebook('
     metadata=md
 )
 
-nb0_py = """# <nbformat>2</nbformat>
+nb0_py = """# coding: utf-8
+# <nbformat>2</nbformat>
 
 # <htmlcell>