Fix one char SMS receiving

Added a ```+1``` in the available's return to avoid that SMS with only 1 char are discarded

Resolve #63 (comment)

Author: Rocketct

src/GSM_SMS.cpp

@@ -149,7 +149,7 @@ int GSM_SMS::available()
       _smsDataEndIndex = _incomingBuffer.length() - 1;
     }
 
-    return (_smsDataEndIndex - _smsDataIndex);
+    return (_smsDataEndIndex - _smsDataIndex) + 1;
   } else {
     _incomingBuffer = "";
   }