Use all of the available space in the buffer returned by ByteBufferFactory to…

Use all of the available space in the buffer returned by ByteBufferFactory to allow the factory to keep a pool of larger than initialsize sized buffers. (#5500)

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -72,7 +72,6 @@ public class FlatBufferBuilder {
         if (initial_size <= 0) {
           initial_size = 1;
         }
-        space = initial_size;
         this.bb_factory = bb_factory;
         if (existing_bb != null) {
           bb = existing_bb;
@@ -82,6 +81,7 @@ public class FlatBufferBuilder {
           bb = bb_factory.newByteBuffer(initial_size);
         }
         this.utf8 = utf8;
+        space = bb.capacity();
     }
 
    /**