Fix fractional(0) returning "0/1" instead of "0"

[Sreekant13]

fractional(0) returns "0/1" instead of "0", even though every other whole number renders bare:

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

The whole-number return branch was gated on whole_number being truthy, so 0 (where whole_number == 0) skipped it and fell through to the fraction branch, producing "0/1". Since numerator == 0 and denominator == 1 already uniquely identifies a whole number (including 0), the whole_number and check is both redundant and wrong for 0. Removing it makes fractional(0) return "0", consistent with fractional(3), fractional(2.0), etc.

Fixes # (no linked issue - small self-contained bug fix)

Changes proposed in this pull request:

• Drop the redundant whole_number and guard in fractional() so 0 is treated as a whole number and returns "0" (previously "0/1").
• Add regression test cases (0, "0") and (0.0, "0") to test_fractional.