Check whetehr the 1st level encoded json is array or not, to
support 1st level array in consul config.
During debug, i suspected the incapability of arrray is caused by
json reader, so i added test for array. I think it makes no harm
to also check that in.
When `consul.StripPrefix(true)` is set, current impl. will pass the
specified prefix (or default prefix) when calling consul api.
However, `prefix` in consul api starts with no leading slash, so
the default prefix (`/micro/config`) doesn't actually work.
I avoid code changes (esp. the one in `util.go`) to eliminate
impact on users who already notice it.
