Datastore tests not running on Windows

[mziccard]

Running datastore tests on Windows Server 2012 leads causes the following error:

java.io.IOException: Cannot run program "cmd": NULL: (The filename, directory name, or volume label syntax is incorrect)

Caused by the NULL: at line:
https://github.com/GoogleCloudPlatform/gcloud-java/blob/master/gcloud-java-datastore/src/test/java/com/google/gcloud/datastore/LocalGcdHelper.java#L171

NUL: (with one L) seems to be a valid null device across Windows versions.

@aozarov shall we close this with #162 (as soon as it's ready) or create a dedicated PR?

[aozarov]

Strange, we did test it on Windows before but maybe this was added later.

Yes, @mziccard thanks for finding it. I think fixing it with #162 is fine.

[aozarov]

Fixed in #162