bug#26260: ~f SRFI-19 format specifier mishandles one-digit seconds value

[Zefram]

The ~f format specifier for SRFI-19's date->string is documented as:

#    ~f     seconds and fractional seconds, with locale
#           decimal point, eg. `5.2'

Let's test that example:

scheme@(guile-user)> (use-modules (srfi srfi-19))
scheme@(guile-user)> (date->string (make-date 200000000 5 34 12 26 3 2017 0) 
"~f")
$1 = "05.2"

That's not the documented format: the doc and the SRFI itself show "5.2"
with no leading padding, but actual behaviour is to zero pad.  There is
much that is ambiguous in the SRFI's specification of ~f, but with that
example it does at least seem clear that there should be no padding there.

-zefram