src: drop less of UTF-8 confidence even with few non-multibyte chars.

Some languages are not meant to have multibyte characters. For instance,
English would typically have none. Yet you can still have UTF-8 English
text (with a few special characters, or foreign words…). So anyway let's
make it less of a deal breaker.

To be even fairer, the whole logics is biased of course and I believe
that eventually we should get rid of these lines of code dropping
confidence on a number of character. This is a ridiculous rule (we base
on our whole logics on language statistics and suddenly we add some
weird rule with a completely random number). But for now, I'll keep this
as-is until we make the whole library even more robust.

src/nsUTF8Prober.cpp

@@ -99,12 +99,13 @@ nsProbingState nsUTF8Prober::HandleData(const char* aBuf, PRUint32 aLen,
 
 float nsUTF8Prober::GetConfidence(int candidate)
 {
-  float unlike = (float)0.99;
-
   if (mNumOfMBChar < 6)
   {
+    float unlike = 0.5f;
+
     for (PRUint32 i = 0; i < mNumOfMBChar; i++)
       unlike *= ONE_CHAR_PROB;
+
     return (float)1.0 - unlike;
   }
   else