Change the default value of fill-tail in Block.of-inline to true

[zeptometer]

According to the document,

%% If [fill-tail] is true, [Inline.fil] is added at the end of the return value.

If fill-tail is false, there may be no Inline.fil in the inline text and sometimes line-break fails to break it into lines. It seems more user-friendly to set fill-tail as true by default because that's what we expect in most of the cases.