SetJSONBody() shouldn't call SetCompressed(true)

[tleyden]

This code doesn't work as expected:

	changesRequest := blip.NewRequest()
	changesRequest.SetCompressed(false)
	changesRequest.SetProfile("changes")
	changesRequest.SetJSONBody(changes)

because SetJSONBody() ends up reverting the compression setting:

func (m *Message) SetJSONBody(value interface{}) error {
	body, err := json.Marshal(value)
	if err == nil {
		m.SetBody(body)
		m.Properties["Content-Type"] = "application/json"
		m.SetCompressed(true)  // <--- reverts earlier setting
	}
	return err
}

This was very confusing and I had to spend time debugging this.

I would like to change this, but before this is changed there would need to do be review of the callers of SetJSONBody() to make sure SetCompressed(true) would need to be enabled where appropriate.

At the very least, the API should be documented to warn users about this.