[Issue 20320] format("%f") leeds to wrong output

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20320

--- Comment #3 from berni44 <bugzilla at d-ecke.de> ---
I meanwhile was able to reproduce this in PR #7264
(https://github.com/dlang/phobos/pull/7264). This PR fixes this for float and
double but not real.

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