Hi,

I ran into some issues while working with the metrics/expand endpoint. This PR addresses this issues.

This should conclude the issue #245

1. The JSON format was not selected by default.

The treejsonFormat was used as the default if none was given, but it was missing as a valid format in the ValidExpandFormat helper.

#Before
curl "http://localhost:8081/metrics/expand?query=node1.value"
unsupported format:

# With the PR
curl "http://localhost:8081/metrics/expand?query=node1.value"
{"results":["node1.value"]}

2. It had the parameters "tz", "from", and "until", which are not described in the Graphite API

I assume this was just a copy-paste artifact from the findHandler, not sure. I removed these parameters.

Regards,
Markus