Cast ByteBuffer instances to Buffer to solve compatibility issue

These changes were applied to ease further work on java9.
For more inforemation see https://jira.mongodb.org/browse/JAVA-2559

DEVSIX-1680

Author: LodrKumquat

io/src/main/java/com/itextpdf/io/source/ByteBufferRandomAccessSource.java

@@ -53,6 +53,7 @@ This file is part of the iText (R) project.
 import java.io.Serializable;
 import java.lang.invoke.MethodHandle;
 import java.lang.reflect.Method;
+import java.nio.Buffer;
 import java.nio.BufferUnderflowException;
 import java.security.AccessController;
 import java.security.PrivilegedAction;
@@ -100,7 +101,7 @@ public int get(long position) throws java.io.IOException {
             throw new IllegalArgumentException("Position must be less than Integer.MAX_VALUE");
         try {
 
-            if (position >= byteBuffer.limit())
+            if (position >= ((Buffer) byteBuffer).limit())
                 return -1;
             byte b = byteBuffer.get((int) position);
             return b & 0xff;
@@ -120,11 +121,11 @@ public int get(long position, byte[] bytes, int off, int len) throws java.io.IOE
         if (position > Integer.MAX_VALUE)
             throw new IllegalArgumentException("Position must be less than Integer.MAX_VALUE");
 
-        if (position >= byteBuffer.limit())
+        if (position >= ((Buffer) byteBuffer).limit())
             return -1;
 
         // Not thread safe!
-        byteBuffer.position((int) position);
+        ((Buffer) byteBuffer).position((int) position);
         int bytesFromThisBuffer = Math.min(len, byteBuffer.remaining());
         byteBuffer.get(bytes, off, bytesFromThisBuffer);
 
@@ -136,7 +137,7 @@ public int get(long position, byte[] bytes, int off, int len) throws java.io.IOE
      * {@inheritDoc}
      */
     public long length() {
-        return byteBuffer.limit();
+        return ((Buffer) byteBuffer).limit();
     }
 
     /**

io/src/main/java/com/itextpdf/io/util/EncodingUtil.java

@@ -46,6 +46,7 @@ This file is part of the iText (R) project.
 import com.itextpdf.io.font.PdfEncodings;
 
 import java.io.UnsupportedEncodingException;
+import java.nio.Buffer;
 import java.nio.CharBuffer;
 import java.nio.charset.CharacterCodingException;
 import java.nio.charset.Charset;
@@ -66,8 +67,8 @@ public static byte[] convertToBytes(char[] chars, String encoding) throws Charac
         CharsetEncoder ce = cc.newEncoder();
         ce.onUnmappableCharacter(CodingErrorAction.IGNORE);
         java.nio.ByteBuffer bb = ce.encode(CharBuffer.wrap(chars));
-        bb.rewind();
-        int lim = bb.limit();
+        ((Buffer) bb).rewind();
+        int lim = ((Buffer) bb).limit();
         int offset = PdfEncodings.UTF8.equals(encoding) ? 3 : 0;
         byte[] br = new byte[lim + offset];
         if (PdfEncodings.UTF8.equals(encoding)) {