Properly set CharsetGroupProber.state to FOUND_IT

Throughout the rest of the chardet code we assume that FOUND_IT means we
can stop looking. Previously the CharsetGroupProber did not set its
state appropriately when a child prober returned FOUND_IT. This
substantially speeds up the chardet for most encodings.

Fixes #202

chardet/charsetgroupprober.py

@@ -73,6 +73,7 @@ def feed(self, byte_str):
                 continue
             if state == ProbingState.FOUND_IT:
                 self._best_guess_prober = prober
+                self._state = ProbingState.FOUND_IT
                 return self.state
             elif state == ProbingState.NOT_ME:
                 prober.active = False