Can now use `CharSequence of String`. Works with

1. [AsciiString]
(http://netty.io/4.1/api/io/netty/util/AsciiString.html)
2. [CharBuffer]
(https://docs.oracle.com/javase/8/docs/api/java/nio/CharBuffer.html)
3. [StringBuilder]
(https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html)

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -373,7 +373,7 @@ public class FlatBufferBuilder {
     * @param s The string to encode.
     * @return The offset in the buffer where the encoded string starts.
     */
-    public int createString(String s) {
+    public int createString(CharSequence s) {
         int length = s.length();
         int estimatedDstCapacity = (int) (length * encoder.maxBytesPerChar());
         if (dst == null || dst.capacity() < estimatedDstCapacity) {