Fix diff? method for newer JunOS versions

[lucasmpb]

There is an issue getting the diff? from a switch when there are no changes to be commited.
On JunOS ~14, ndev.rpc.get_configuration( :compare=>'rollback', :rollback=> rollback_id.to_s )returns an xml response:

<configuration-information>
<configuration-output>
</configuration-output>
</configuration-information>

But on JunOS ~17, the same method call, returns nil making the subsequent line to fail with undefined method 'xpath' for nil:NilClass

Adding :format => 'text' to the call seems to make the behavior consistent across different JunOS versions

PS: I took the idea from the more up to date python library https://github.com/Juniper/py-junos-eznc/blob/ce914d19030f51067b0c02c72fd69557f26c49e9/lib/jnpr/junos/utils/config.py#L225

[vnitinv]

@lucasmpb why do we need to add format? What changed in the newer version of Junos?

[lucasmpb]

Sorry @vnitinv I should have explained myself. I added a description to the pull request.

[vnitinv]

@lucasmpb I agree here it should be format text only. But scared if this change will break any existing user code. Will discuss with team.

@ydnath Can you review this. I think this change should go in.

[ydnath]

Checked and does not impact north bound api