Small fix to be compatible with < 1.7.8

Author: lvca

src/main/java/com/orientechnologies/orient/etl/extractor/OJsonExtractor.java

@@ -74,7 +74,7 @@ public void extract(final Reader iReader) {
     super.extract(iReader);
     jsonReader = new OJSONReader(reader);
     try {
-      first = jsonReader.nextChar();
+      first = (char) reader.read();
       if (first == '[')
         first = null;
       else if (first == '{')