Potential fix for test_utf8 not working on some data frames

Hi there,

First Git pull request - hopefully I didn't mess this up. I noticed that `test_utf8` failed on some data frames, giving this error:

```
Error in charToRaw(x) : argument must be a character vector of length 1
```

The error is pretty intuitive, although I still couldn't find what was causing this. All the column names are strings so I've no idea what's happening there...

In my fork of the repo I added a file in `inst/extdata` called `minimal.csv`, if you load this CSV into R and run `test_utf8` on it you will get the error. There was also another dataset I used, which worked fine, but when I reduced its dimensions to be the same as `minimal.csv`, I got the same error.

Despite not quite finding what the problem was, I decided to re-write one of the lines in `test_utf8.R`. Also no warnings need to be suppressed for this new line. :)

Cheers