Handle already singular compound leaders

[deepakganesh78]

Fixes #222.

_handle_long_compounds passed the result of singularizing the leading noun directly into str.join. When that noun was already singular, _sinoun returned False, causing singular_noun("pair of scissors") to raise a TypeError.

This uses the original candidate whenever the inflection result is not a string. The plural path is unchanged because _plnoun returns strings, while already-singular compound leaders now remain intact. A regression test and towncrier fragment are included.

Validation:

• python -m pytest -q — 216 passed, 16 xfailed
• tests/test_compounds.py — 10 passed
• mypy reports no error on the changed compound logic; the remaining three errors in inflect/__init__.py are pre-existing