A few updates to the Dart implementation.

- Use fixed contentLength instead of chunked encoding.
- Simplify (and remove some list overhead) of DB tests.
- Add implementation for 'queries'.
- Renamed 'update' to 'updates'.
- Renamed '/' to '/json'.
- Override existing Server header.
Update benchmark_config as well.
Merge pull request #379 from skabet/master

Update Dart implementation with simplifications and minor speedups.