NaN results from monitoring metrics cause JSON parser error

```ruby
r,p,data = client.do_get '/api/clouds/####/instances/######/monitoring_metrics/load:load/data?start=-60&end=0'
JSON::ParserError: 209: unexpected token at 'NaN,NaN],"variable":"shortterm"},{"points":[0.01,0.01,0.01,0.01,0.018,NaN,NaN],"variable":"longterm"},{"points":[0.03,0.038,0.04,0.04,0.064,NaN,NaN],"variable":"midterm"}],"end":"2013/08/06 22:39:50 +0000"}'
```

Probably something at an API level that needs to be fixed/handled in the raw response body, but perhaps we can gsub the response body before JSON.parse as a workaround?