drop_exists() is case-sensitive

`drop_exists()` is case-sensitive. This means that it will incorrectly return `FALSE` in the following scenario:

1. Alice creates (in her Dropbox) a file named `/Foo/bar/baz.txt`.
2. Alice shares `/Foo/` with Bob.
3. Bob renames (on his filesystem) the subfolder `bar/` to `Bar/`.
4. Bob invokes `drop_exists("/Foo/Bar/baz.txt")`, expecting the result to be `TRUE`.

Following up with a pull request.

Start by locating the drop_exists() entry point and reproducing the case-only folder rename described in the issue. Trace how Dropbox paths are compared, then verify that the example returns TRUE without changing behavior for genuinely different paths.

Written by the indexing model from the issue text.