Description:
------------
In RFC 822 all header names are defined as case independent. Therefore it would be useful if a mode exists that lowercases all header names and also merge multiple headers with the lowercased name in iconv_mime_decode_header().

It is possible to do that after iconv_mime_decode_header(), but merging multiple headers needs a fair amount of code, which is also already in iconv_mime_decode_header().

Reproduce code:
---------------
$message = <<<ETX
received: line 1
RECEIVED: line 2
Received: line 3
Subject: test
ETX;

var_dump(iconv_mime_decode_headers($message, ICONV_MIME_DECODE_LOWER));

Expected result:
----------------
array(2) {
  ["received"]=>
  array(4) {
    [0]=>
    string(6) "line 1"
    [1]=>
    string(6) "line 2"
    [2]=>
    string(6) "line 3"
    [3]=>
    string(6) "line 4"
  }
  ["subject"]=>
  string(4) "test"
}

Actual result:
--------------
// currently not possible - with the default mode:
array(4) {
  ["received"]=>
  string(6) "line 1"
  ["RECEIVED"]=>
  string(6) "line 2"
  ["Received"]=>
  array(2) {
    [0]=>
    string(6) "line 3"
    [1]=>
    string(6) "line 4"
  }
  ["Subject"]=>
  string(4) "test"
}