imf_data request fails

Running the example **Simple Country-Time-Variable**

```
library(imfr)

real_ex <- imf_data(database_id = 'IFS', indicator = 'EREER_IX',
country = c('CN', 'GB'), freq = 'A',
start = 2013, end = current_year())
```

I get

```
Downloading: 150 kB Error: Unable to download series.
```
Windows ` sessionInfo()`

```
R version 3.4.4 (2018-03-15)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

Matrix products: default

locale:
[1] LC_COLLATE=Estonian_Estonia.1257 LC_CTYPE=Estonian_Estonia.1257 LC_MONETARY=Estonian_Estonia.1257 LC_NUMERIC=C
[5] LC_TIME=Estonian_Estonia.1257

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

other attached packages:
[1] imfr_0.1.5

loaded via a namespace (and not attached):
[1] Rcpp_0.12.17 dplyr_0.7.5 assertthat_0.2.0 R6_2.2.2 jsonlite_1.5 magrittr_1.5 httr_1.3.1 pillar_1.2.3
[9] rlang_0.2.1 curl_3.2 bindrcpp_0.2.2 tools_3.4.4 glue_1.2.0 purrr_0.2.5 yaml_2.1.19 compiler_3.4.4
[17] pkgconfig_2.0.1 bindr_0.1.1 tidyselect_0.2.4 tibble_1.4.2
```

Running the same code snippet on OSX gives the same error.

OSX `sessionInfo()`

```
R version 3.5.0 (2018-04-23)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.5

Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

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

other attached packages:
[1] imfr_0.1.5

loaded via a namespace (and not attached):
[1] Rcpp_0.12.17 dplyr_0.7.5 assertthat_0.2.0 R6_2.2.2 jsonlite_1.5 magrittr_1.5 httr_1.3.1 pillar_1.2.3 rlang_0.2.1
[10] curl_3.2 bindrcpp_0.2.2 tools_3.5.0 glue_1.2.0.9000 purrr_0.2.5 yaml_2.1.19 compiler_3.5.0 pkgconfig_2.0.1 bindr_0.1.1
[19] knitr_1.20 tidyselect_0.2.4 tibble_1.4.2

```

Start by reproducing the Simple Country-Time-Variable example through the imf_data entry point and inspect the request and response handling for the IMF RESTful JSON API. Compare the failing response with the expected series download and determine what behavior should be corrected; done means the example downloads successfully or reports the underlying failure clearly.

Written by the indexing model from the issue text.