Error in drop_download: Conflict (HTTP 409)

Hi,

I had a working shiny app using rdrop2 for sometime.
When I tried to use it today the app did not retrieved the files that are within DropBox.

Back to R to study what happened I found that that drop_get is deprecated and that I received now the following error:

```r
Downloading: 94 B Warning: Error in drop_download: Conflict (HTTP 409).
Stack trace (innermost first):
63:
62: stop
61: httr::stop_for_status
60: drop_download
59: eval [/My/DropBox/File/Path/app.R#404]
58: eval
57: withProgress
56: observerFunc [/My/DropBox/File/Path//app.R#399]
1: runApp
```

I changed drop_get() to drop_download() and I have read posts on 409 error messages but I have not came up with a solution.
Help on this is really appreciated. Thanks

```r
> sessionInfo()
R version 3.4.2 (2017-09-28)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.3 LTS

Matrix products: default
BLAS: /usr/lib/libblas/libblas.so.3.6.0
LAPACK: /usr/lib/lapack/liblapack.so.3.6.0

locale:
[1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C LC_TIME=es_PR.UTF-8
[4] LC_COLLATE=en_US.UTF-8 LC_MONETARY=es_PR.UTF-8 LC_MESSAGES=en_US.UTF-8
[7] LC_PAPER=es_PR.UTF-8 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=es_PR.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] httr_1.3.1 shiny_1.0.5 rdrop2_0.8.1.9999

loaded via a namespace (and not attached):
[1] Rcpp_0.12.13 digest_0.6.12 mime_0.5 R6_2.2.2 xtable_1.8-2 jsonlite_1.5
[7] magrittr_1.5 curl_3.0 tools_3.4.2 httpuv_1.3.5 yaml_2.1.14 rsconnect_0.8.5
[13] compiler_3.4.2 htmltools_0.3.6 openssl_0.9.7
```

Start with the drop_download entry point and the failing call at app.R#404, then compare it with the deprecated drop_get usage and the reported HTTP 409 response. Reproduce the download against Dropbox and use the stack trace to identify the failing request; done means the app retrieves the files successfully.

Written by the indexing model from the issue text.