error 500 on luarocks.org/upload

Nobody has claimed this yet.

This is from the web interface, but I got the same thing (minus traceback) from the CLI.

```
Error

...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:668: ...luarocks/.luarocks/share/lua/5.1/cloud_storage/oauth.lua:46: Expected value but found T_END at character 1
stack traceback:
[C]: in function 'decode'
...luarocks/.luarocks/share/lua/5.1/cloud_storage/oauth.lua:46: in function 'refresh_access_token'
...luarocks/.luarocks/share/lua/5.1/cloud_storage/oauth.lua:33: in function 'get_access_token'
...uarocks/.luarocks/share/lua/5.1/cloud_storage/google.lua:203: in function '_headers'
...uarocks/.luarocks/share/lua/5.1/cloud_storage/google.lua:221: in function 'put_file_string'
./helpers/uploaders.lua:104: in function 'do_rockspec_upload'
./helpers/uploaders.lua:161: in function 'handle_rockspec_upload'
./app.lua:112: in function <./app.lua:110>

Traceback

stack traceback:
[C]: in function 'error'
...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:668: in function 'handler'
...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:402: in function 'resolve'
...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:411: in function <...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:409>
[C]: in function 'xpcall'
...e/luarocks/.luarocks/share/lua/5.1/lapis/application.lua:409: in function 'dispatch'
/home/luarocks/.luarocks/share/lua/5.1/lapis/nginx.lua:205: in function
```

The files I tried to upload:
https://alloyed.me/shared/loadconf-0.3.0-1.src.rock
https://alloyed.me/shared/loadconf-0.3.0-1.rockspec

Start with cloud_storage/oauth.lua at refresh_access_token, then trace the upload path through cloud_storage/google.lua and helpers/uploaders.lua. Reproduce the web or CLI upload using the linked loadconf files, and consider the issue done when the upload no longer returns the JSON decode error or HTTP 500.

Written by the indexing model from the issue text.