test: return the input string if no stemmer is available.

tests/stem_word.result

@@ -12,3 +12,6 @@ deur boekjes stuiter stuiter auto'
 SELECT stem_word("english", "cars#bars#opening");
 stem_word("english", "cars#bars#opening")
 car bar open
+SELECT stem_word("xyz", "Xxx Yyys Zzzer");
+stem_word("xyz", "Xxx Yyys Zzzer")
+Xxx Yyys Zzzer

tests/stem_word.test

@@ -18,4 +18,5 @@ error 1123; SELECT stem_word();
 error 1123; SELECT stem_word("dutch");
 SELECT stem_word("dutch", "deuren");
 SELECT stem_word("dutch", "deuren boekjes;stuiter stuiteren auto's");
-SELECT stem_word("english", "cars#bars#opening");
\ No newline at end of file
+SELECT stem_word("english", "cars#bars#opening");
+SELECT stem_word("xyz", "Xxx Yyys Zzzer");