singular_noun on word ending in "s" removes s.

```

import inflect as Inflect
inflect = Inflect.engine()
inflect.singular_noun("car") # get False, as expected
inflect.singular_noun("mass") # get "mas", expected False

```

Is this behavior a bug? singular_noun() is supposed to return False when the word is already singular, and I can find no reference to suggest "mass" is the plural of "mas", which is itself the plural of "ma".

Start at the singular_noun() entry point and reproduce the reported results for "car" and "mass". Trace the word-ending-in-s handling and add regression coverage so the behavior for an already singular word matches the documented expectation; the issue names no specific file or test.

Written by the indexing model from the issue text.