Ensure encoding header gets written at start of file

Check whether the file didn't exist beforehand or if we're
not appending (in which case the file gets written anew anyways.

IPython/core/magics/code.py

@@ -81,7 +81,8 b' class CodeMagics(Magics):'
         fname, codefrom = unquote_filename(args[0]), " ".join(args[1:])
         if not fname.endswith((u'.py',u'.ipy')):
             fname += ext
-        if os.path.isfile(fname) and not force and not append:
+        file_exists = os.path.isfile(fname)
+        if file_exists and not force and not append:
             try:
                 overwrite = self.shell.ask_yes_no('File `%s` exists. Overwrite (y/[N])? ' % fname, default='n')
             except StdinNotImplementedError:
@@ -97,7 +98,7 b' class CodeMagics(Magics):'
             return
         out = py3compat.cast_unicode(cmds)
         with io.open(fname, mode, encoding="utf-8") as f:
-            if not append:
+            if not file_exists or not append:
                 f.write(u"# coding: utf-8\n")
             f.write(out)
             # make sure we end on a newline