Preserve default headers, fixing openwrt test

This problem was encountered when running
```bash
python dumpgenerator.py --get-wiki-engine http://wiki.openwrt.org
```
The server required an "Accept" HTTP header, which was being removed when setting the user-agent. It works now. Of course, it should be determined whether this behaviour was intentional or not before merging.
This **needs** to be tested before being merged.  It passes all the unit tests, but it might not work in all cases.