Do not send Expect: 100-continue header unless explicitly specified

```
IOR currently sends this by default for PUT/POST requests, but some servers 
setups don't handle this header properly. Add a setting to turn it off, or 
simply don't send it unless it is explicitly added to the headers.
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 22 Aug 2014 at 7:43
