'defnoun' doesn't seem to work when the word is a part of a string passed

Originally reported by @aMiss-aWry in #161:

I can fix the specific words with defnoun('jeans', 'jeans'), which corrects the result for an isolated string 'jeans' -> 'jeans'.

However, if I try to run plural_noun on 'blue jeans' instead of 'jeans' it then breaks again, returning 'blue jeanss'. I understand this is because 'blue jeans' appears to be a different string from 'jeans', but perhaps it should check the rules for the part of the string it is altering?