Non-ASCII attachment filename decoding

I had a problem downloading message parts with headers containing non-ASCII characters (attachment name/filename, to be exact).

Here is example part of received message:
```
--b1_21c078e69d0b21d50fd733085ceb8767
Content-Type: application/vnd.ms-excel; name="=?UTF-8?Q?Prost=C5=99eno=5F2014=5Fposledn=C3=AD_voln=C3=A9_term=C3=ADny.xls?="; charset="UTF-8"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="=?UTF-8?Q?Prost=C5=99eno=5F2014=5Fposledn=C3=AD_voln=C3=A9_term=C3=ADny.xls?="

0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAACAAAAwgAAAAAA
AAAAEAAA/v///wAAAAD+////AAAAAMAAAADBAAAA////////////////////////////////
...
```

This part gets decoded by ```imap_fetchstructure``` like this:
```php
stdClass Object(
    [type] => 3
    [encoding] => 3
    [ifsubtype] => 1
    [subtype] => VND.MS-EXCEL
    [ifdescription] => 0
    [ifid] => 0
    [bytes] => 137524
    [ifdisposition] => 1
    [disposition] => ATTACHMENT
    [ifdparameters] => 1
    [dparameters] => Array(
        [0] => stdClass Object(
            [attribute] => FILENAME
            [value] => =?ISO-8859-2?Q?Prost=F8eno=5F2014=5Fposledn=ED_voln=E9_term=EDny.xls?=
        )
    )

    [ifparameters] => 1
    [parameters] => Array(
        [0] => stdClass Object(
            [attribute] => CHARSET
            [value] => UTF-8
        )
        [1] => stdClass Object(
            [attribute] => NAME
            [value] => =?ISO-8859-2?Q?Prost=F8eno=5F2014=5Fposledn=ED_voln=E9_term=EDny.xls?=
        )
    )
)

```

To get the original value, I need to apply ```imap_utf8``` and ```imap_mime_header_decode``` functions. After that, a get the right filename ```Prostřeno_2014_poslední volné termíny.xls```.