Fixed case sensitive HTTP header lookup in TransferManager. 

Fixed bug with case insensetive "Content-Disposition" HTTP header processing.
Steps to reproduce:
1. Open URL [http://www.opera.com/ru/computer/thanks?ni=stable&os=windows];
2. Wait till download starts.
Actual result: Downloading of a file named "windows" starts
Expected result: Downloading of "Opera_NI_stable.exe" starts

The proble is, that ```reply->rawHeaderList().contains(...)``` makes search case sensitive. Replaced with ```reply->hasRawHeader(...)```

P.S. Added default "All files (*)" filter for file save dialog.