Fix fractional() emitting degenerate output when the fraction rounds to a whole

[semx]

Noticed while probing edge cases: fractional() emits degenerate output when the fractional part rounds to a whole number.

>>> import humanize
>>> humanize.fractional(2.9999999)
'2 1/1'        # expected '3'
>>> humanize.fractional(0.9999999)
'1/1'          # expected '1'
>>> humanize.fractional(0)
'0/1'          # expected '0'

Cause

After frac = Fraction(number - whole_number).limit_denominator(1000), when the fractional part reduces to a whole number the denominator is 1 (numerator ∈ {-1, 0, 1}). The existing special case only handled numerator == 0 (a plain integer input like 1.0), so:

• numerator == 1 (the fraction rounded up to 1/1) fell through to the mixed-fraction branch → "2 1/1";
• fractional(0) produced "0/1".

Changes proposed in this pull request:

• Fold any whole-valued fractional part (denominator == 1) into the integer part, so the result reads as a normal integer.
• Add regression tests for 0, 0.0, 2.9999999, 0.9999999, -2.9999999.

All existing tests pass (705 passed); ruff and black clean.