Issue with folder names with capital letter ?

```r
drop_dir('AFL_Resources/Model/db')
Source: local data frame [6 x 5]

path mime_type root bytes modified

1 /AFL_Resources/model/db/afl.csv text/csv dropbox 354595 Mon, 11 May 2015 17:55:34 +0000
2 /AFL_Resources/model/db/lookup_stadium.csv text/csv dropbox 2234 Mon, 11 May 2015 17:55:34 +0000
3 /AFL_Resources/model/db/stadium.csv text/csv dropbox 924 Mon, 11 May 2015 17:55:34 +0000
4 /AFL_Resources/model/db/team_stadium.csv text/csv dropbox 1689 Mon, 11 May 2015 17:55:34 +0000
5 /AFL_Resources/model/db/team.csv text/csv dropbox 638 Mon, 11 May 2015 17:55:34 +0000
6 /AFL_Resources/model/db/teamStadium.csv text/csv dropbox 3451 Mon, 11 May 2015 17:55:34 +0000

aflData <- drop_read_csv(file = "AFL_Resources/Model/db/afl.csv")
File not found on Dropbox
```

Capital letter seems to be the problem as if I rename Model in model then it works fine

```r
drop_dir('AFL_Resources/model/db')
path mime_type root bytes modified
1 /AFL_Resources/model/db/afl.csv text/csv dropbox 354595 Mon, 11 May 2015 17:55:34 +0000
2 /AFL_Resources/model/db/lookup_stadium.csv text/csv dropbox 2234 Mon, 11 May 2015 17:55:34 +0000
3 /AFL_Resources/model/db/stadium.csv text/csv dropbox 924 Mon, 11 May 2015 17:55:34 +0000
4 /AFL_Resources/model/db/team_stadium.csv text/csv dropbox 1689 Mon, 11 May 2015 17:55:34 +0000
5 /AFL_Resources/model/db/team.csv text/csv dropbox 638 Mon, 11 May 2015 17:55:34 +0000
6 /AFL_Resources/model/db/teamStadium.csv text/csv dropbox 3451 Mon, 11 May 2015 17:55:34 +0000

aflData <- drop_read_csv(file = "/AFL_Resources/model/db/afl.csv")
/tmp/RtmpYIbY9x/afl.csv on disk 354.595 KB
```

Reproduce the failure using drop_dir and drop_read_csv with the capitalized AFL_Resources/Model/db path, then compare it with the lowercase model path that succeeds. Trace the path handling used by these entry points; done means files under a capitalized Dropbox folder can be listed and read successfully.

Written by the indexing model from the issue text.