Made reading read-only ByteBuffers work.

Also added new constructor that allows ByteBuffer reuse.

Change-Id: I9c20ea96c67533066461f4e23b0d03b9b47cd068
Tested: on OS X.

tests/JavaTest.java

@@ -103,9 +103,12 @@ class JavaTest {
         }
 
         // Test it:
-
         TestBuffer(fbb.dataBuffer(), fbb.dataStart());
 
+        // Make sure it also works with read only ByteBuffers. This is slower, since
+        // creating strings incurs an additional copy (see Table.__string).
+        TestBuffer(fbb.dataBuffer().asReadOnlyBuffer(), fbb.dataStart());
+
         System.out.println("FlatBuffers test: completed successfully");
     }