Fix header encoding of long headers when using euc-jp and shift_jis.

When a header was long enough to need to be split across lines, the
input charset name was used instead of the output charset name in
the encoded words.  This make a difference only for the two charsets
above.
diff --git a/Lib/email/charset.py b/Lib/email/charset.py
index 8591527..24d5545 100644
--- a/Lib/email/charset.py
+++ b/Lib/email/charset.py
@@ -321,7 +321,7 @@
         codec = self.output_codec or 'us-ascii'
         header_bytes = _encode(string, codec)
         encoder_module = self._get_encoder(header_bytes)
-        encoder = partial(encoder_module.header_encode, charset=str(self))
+        encoder = partial(encoder_module.header_encode, charset=codec)
         # Calculate the number of characters that the RFC 2047 chrome will
         # contribute to each line.
         charset = self.get_output_charset()