Fixes inconsistent API typing for sub_type

[leucos]

UptimeRobot API returns inconsistent types on sub_type (just like keyword_type).
This is addressed using interface{} instead of int

A test case has been added.

Should fix #5

[bitfield]

Fantastic! Thanks for the PR.

As I expect you know, the problem with keyword_type and sub_type is that when they're set, they're integers, but when they're not set, they're returned as an empty string. I suspect the JSON parsing libraries in other languages are less strict than Go (see golang/go#22182), and can cope with this, but nonetheless, the API should return a consistent type (probably null if it's unset).

I've contacted Uptime Robot about this, but evidently they haven't fixed it yet.