Fix sending Transmission content with template_id.

[Tang8330]

Current logic is flawed in 2 ways.

1. Iterating over a Golang map is not deterministic
2. Other data types are actually valid to pass into message such as CC and BCC which are map[string]string types. Logic would early return when it found template_id based on [1], but because iterating keys over a map is not deterministic, it would fetch CC and then err out because it's map[string]string not string

[chrsmith]

I came here to report the same problem and propose a similar fix. @Tang8330 were you able to find a workaround so you can reliably send your transmissions? Otherwise calls to transmission.ParseContent(...) will randomly fail. (Due to a map's keys always being returned in an "indeterminate" ordering.)

It seems like the only(?) workaround for using stored templates with Sparkpost is to simply not use this Golang library?

[Tang8330]

@chrsmith I fixed the problem on my fork and we're using that in Production.