Fix format ~F (2nd) and update tests

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I fixed an error which occurs when the inexact number string doesn't
contain a dot.
(Gauche and Sagittarius don't produce such a string, but Racket does.)

I created a pull request as follows.
https://github.com/scheme-requests-for-implementation/srfi-48/pull/2

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<div dir=3D"ltr"><div>I fixed an error which occurs when the inexact number=
 string doesn&#39;t contain a dot.</div><div>(Gauche and Sagittarius don&#3=
9;t produce such a string, but Racket does.)</div><div><br></div><div><div>=
I created a pull request as follows.</div></div><div><div><a href=3D"https:=
//github.com/scheme-requests-for-implementation/srfi-48/pull/2">https://git=
hub.com/scheme-requests-for-implementation/srfi-48/pull/2</a></div></div><d=
iv><br></div></div>

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