formpost: use semicolon in multipart/mixed

Not comma, which is an inconsistency and a mistake probably inherited
from the examples section of RFC1867.

This bug has been present since the day curl started to support
multipart formposts, back in the 90s.

Reported-by: Rob Davies
Bug: http://curl.haxx.se/bug/view.cgi?id=1333

lib/formdata.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -1227,7 +1227,7 @@ CURLcode Curl_getformdata(struct SessionHandle *data,
       }
 
       result = AddFormDataf(&form, &size,
-                            "\r\nContent-Type: multipart/mixed,"
+                            "\r\nContent-Type: multipart/mixed;"
                             " boundary=%s\r\n",
                             fileboundary);
       if(result)

tests/data/test1133

@@ -40,7 +40,7 @@ foo
 # Verify data after the test has been "shot"
 <verify>
 <strip>
-^(User-Agent:|Content-Type: multipart/form-data;|Content-Type: multipart/mixed, boundary=|-------).*
+^(User-Agent:|Content-Type: multipart/form-data;|Content-Type: multipart/mixed; boundary=|-------).*
 </strip>
 <protocol>
 POST /we/want/1133 HTTP/1.1
@@ -71,7 +71,7 @@ foo
 
 ------------------------------24e78000bd32
 Content-Disposition: form-data; name="file3"
-Content-Type: multipart/mixed, boundary=----------------------------7f0e85a48b0b
+Content-Type: multipart/mixed; boundary=----------------------------7f0e85a48b0b
 
 Content-Disposition: attachment; filename="test1133,a\"nd;.txt"
 Content-Type: m/f