body_encode(): Fixed typo reported by Chris Lawrence, closing SF bug

#625509.  This isn't a huge problem because at the moment there are no
built-in charsets for which header_encoding is QP but body_encoding is
not.

Lib/email/Charset.py

@@ -348,7 +348,7 @@ class Charset:
         # 7bit/8bit encodings return the string unchanged (module conversions)
         if self.body_encoding is BASE64:
             return email.base64MIME.body_encode(s)
-        elif self.header_encoding is QP:
+        elif self.body_encoding is QP:
             return email.quopriMIME.body_encode(s)
         else:
             return s