Fix response filepath when contains directory

[mkcor]

This PR addresses issue #26.

What's posted is actually fine (resulting tree in plotly account is what you expect online).

Value kwargs$filename is preserved under toJSON() but then under postForm() some parsing issue generates the double directory prepending... Which in turns propagates to fromJSON() -- I'm talking about https://github.com/ropensci/plotly/blob/master/R/plotly.R#L107

That's why I suggest overwriting resp$filename with the initial kwargs$filename so the returned response is what you expect -- no impact on what's actually sent to plotly (as the web platform).

Note that the resulting plotly plot is private by default -- is this what we expect?

/cc @sckott @chriddyp

[mkcor]

@theengineear bambi eyes

[theengineear]

Looks reasonable and it has the tests. 🎉

[mkcor]

Thanks!

Updating version number and NEWS file...