Temporarily compare buffer data lengths, not capacity

After slicing buffers, we can end up with 2 buffers having the same len and data, but not capacity.
Such buffers fail the equality test.

CC @sunchao  jorgecarleitao as you've worked on this.
Perhaps we can find a suitable definition of equality, as we're otherwise failing integration tests because of this.

rust/arrow/src/buffer.rs

@@ -70,7 +70,7 @@ struct BufferData {
 
 impl PartialEq for BufferData {
     fn eq(&self, other: &BufferData) -> bool {
-        if self.capacity != other.capacity {
+        if self.len != other.len {
             return false;
         }