drop_auth() Throws to Local Host, Cannot Load Page

After "Allowing" rdrop2 on Dropbox, I am re-directed to a local host page which cannot be loaded.

The URL, prior to clicking "Allow" shows the localhost re-direct:

https://www.dropbox.com/1/oauth2/authorize?client_id=mmhfsybffdom42w&scope=&redirect_uri=http%3A%2F%2Flocalhost%3A1410&response_type=code&state=axxpkm4iIq

After clicking allow I'm re-direct to http://localhost:1410 which cannot be loaded.

I confirm that I am logged into my Dropbox account.

I am using v0.7 of rdrop2, installed initially from CRAN and then from GitHub for the latest version.

My development environment is RStudio Server running on a remote Digital Ocean droplet.

My session info is:

```
> devtools::session_info()
Session info -------------------------------------------------------------------------------------------------------------------------------------------------------------------
setting value
version R version 3.2.2 (2015-08-14)
system i486, linux-gnu
ui RStudio (0.98.1083)
language (EN)
collate C
tz

Packages -----------------------------------------------------------------------------------------------------------------------------------------------------------------------
package * version date source
DBI 0.3.1 2014-09-24 CRAN (R 3.2.2)
R6 2.1.1 2015-08-19 CRAN (R 3.2.2)
Rcpp 0.12.1 2015-09-10 CRAN (R 3.2.2)
assertthat 0.1 2013-12-06 CRAN (R 3.2.2)
chron 2.3-47 2015-06-24 CRAN (R 3.2.2)
curl 0.9.3 2015-08-25 CRAN (R 3.2.2)
data.table * 1.9.6 2015-09-19 CRAN (R 3.2.2)
devtools 1.8.0 2015-05-09 CRAN (R 3.2.2)
digest 0.6.8 2014-12-31 CRAN (R 3.2.2)
dplyr * 0.4.3 2015-09-01 CRAN (R 3.2.2)
git2r 0.11.0 2015-08-12 CRAN (R 3.2.2)
htmltools 0.2.6 2014-09-08 CRAN (R 3.2.2)
htmlwidgets 0.5 2015-06-21 CRAN (R 3.2.2)
httpuv 1.3.3 2015-08-04 CRAN (R 3.2.2)
httr 1.0.0 2015-06-25 CRAN (R 3.2.2)
jsonlite 0.9.17 2015-09-06 CRAN (R 3.2.2)
lpSolve * 5.6.12 2015-09-01 CRAN (R 3.2.2)
magrittr 1.5 2014-11-22 CRAN (R 3.2.2)
memoise 0.2.1 2014-04-22 CRAN (R 3.2.2)
mime 0.4 2015-09-03 CRAN (R 3.2.2)
rdrop2 * 0.7 2015-09-22 Github (karthik/rdrop2@15e59ef)
rhandsontable * 0.2 2015-08-25 CRAN (R 3.2.2)
rstudio 0.98.1083 2015-09-08 local
rstudioapi 0.3.1 2015-04-07 CRAN (R 3.2.2)
rversions 1.0.2 2015-07-13 CRAN (R 3.2.2)
shiny * 0.12.2 2015-08-05 CRAN (R 3.2.2)
shinyBS * 0.61 2015-03-31 CRAN (R 3.2.2)
stringi 0.5-5 2015-06-29 CRAN (R 3.2.2)
stringr 1.0.0 2015-04-30 CRAN (R 3.2.2)
xml2 0.1.2 2015-09-01 CRAN (R 3.2.2)
xtable * 1.7-4 2014-09-12 CRAN (R 3.2.2)
```

Start with the drop_auth() entry point and reproduce the OAuth flow in the reported RStudio Server environment. Inspect how the Dropbox authorization callback uses localhost:1410; done means authorization returns successfully and the page loads instead of failing at localhost.

Written by the indexing model from the issue text.