Add preemptive instance request fields to server

[DigitallyBorn]

In addition to adding the preemptive instance fields (aka spot instance), I simplified the url/parameter building a little bit to make it easier to read/expand.

There was also a bug detecting the content type. Fixing that allowed the fog-core library to automatically deserialize some responses, so we don't have to explicitly decode responses here anymore.

Also, claiming a little credit for myself for bragging rights.