CSV-279: Optimize Lexer Delimiter Check for One Character Delimiter (#163)

Author: belugabehr

src/main/java/org/apache/commons/csv/Lexer.java

@@ -123,6 +123,9 @@ boolean isDelimiter(final int ch) throws IOException {
         if (ch != delimiter[0]) {
             return false;
         }
+        if (delimiter.length == 1) {
+          return true;
+        }
         final int len = delimiter.length - 1;
         final char[] buf = reader.lookAhead(len);
         for (int i = 0; i < len; i++) {