Simple csv file can't be parsed correctly because of Sniffer() parameters

Greetings,

First of all, thank you for the work you are doing on this library.

I have the following CSV file, where the delimiter is a colon `:` :
```csv
test:test2
value1:value2
```

I'm now using the `tablib` library to import it:
```python
tmp = open('test.csv', 'r')
tmp2 = tablib.Dataset().load(tmp, format='csv')
```

But the file is wrongly parsed, because the `csv` library did not sniffed the delimiter correctly:
```
test:test2
-------------
value1:value2
```

I saw that you are passing `2048 bytes` of samples to be sniffed (https://github.com/jazzband/tablib/blob/4bbb35a4103fea2e091c15314f0a1c53fc9613cb/src/tablib/formats/_csv.py#L58), I now tested to manually sniff the CSV using only the first line:
```python
dialect = csv.Sniffer().sniff(tmp.readline())
```

The delimiter is now indeed recognized as a colon `:`.

My question is: shouldn't you pass the first line to sniff only (using `readline()`) ? Or maybe fallback on the actual behaviour only if the headers is not present in the file ?

Related issue: #502

Regards