Clipboard Copy Off-by-one Column in BeakerX table

Using the Groovy kernel I make a table like this:

```groovy
[
[foo: 1, bar:2],
[foo: 10, bar:20]
] * 5
```
This displays like so:


I then select some values from the `foo` column and choose "Copy" from the menu:


Choosing Copy:


Then I paste the values into the next cell - but only empty values are pasted:


However if I also select the column to the left:


Then it pastes the values I wanted originally:


The same behavior happens for the `bar` column. To copy the values from `bar` I have to also select the values from `foo`, which then copies only the `bar` column and not `foo` and `bar`.

Expected behaviour: the column I select should be copied when only that column is selected.

I am using Chrome with a checkout and build from master (commit #774fa4)