Fix regression introduced by 78fb5c7

This is in response to @dpocock 's comment [here](https://github.com/ngzhian/jmxetric/commit/78fb5c7566a22c9b30eaeeb1198c85e2e6d0297e#commitcomment-6552086). This change fixes the regression, the reported bug is [here](https://github.com/ganglia/jmxetric/pull/17)