Show bitfield for unknown length download in aria2.tellStatus RPC method

Generally, bitfield is not available for download whose total length
is unknown.  We create bitfield when download is completed (usually
connection EOF) so that we can use it to show additional info in RPC
aria2.tellStatus response.  Specifically, bitfield is now shown.  And
completedLength under files key (or completedLength in
aria2.getFiles() response) is correctly shown.