%set_env should ensure environment variables are bytes on Python 2

Should fix test failures on Python 2 + Windows

IPython/core/magics/osm.py

@@ -413,7 +413,7 class OSMagics(Magics):
             err = "refusing to set env var with whitespace: '{0}'"
             err = err.format(val)
             raise UsageError(err)
-        os.environ[var] = val
+        os.environ[var] = py3compat.cast_bytes_py2(val)
         print('env: {0}={1}'.format(var,val))
 
     @line_magic