Fix issues with successive calls to .create() or .update() on resources.
The create and update methods had a parameter data which had a default value

of {}. In the body of these methods, we used data.update(...), which would

write to that default value. This means that successive calls would include

data that wasn't intended.
This was hit when using 'rbt post -u CLN' to update a public review request

with a perforce repository. We'd update the review request resource with

'changenum', and then try to update the draft with 'public', but that would end

up also sending 'changenum' to the draft resource, which doesn't have that

attribute.

Reproduced the case in the bug. After this fix, I can use 'rbt post -u CLN' to

update and publish a review request all in one go.