Spent some time trying to complete this tonight. Please review as accurately as possible. All of my types are from reading the code. I based some off of the JSON Schema object but also included anything written in the documentation too. There were a couple any types I'd like to see if we can make more specific, as well as some general questions I have:

• Is the port property on the InjectOptions.url type of string | number?
• What is query? Could this be typed as string?
• What types can payload be? I've implemented it as a generic for now but I think we can scope this to an union type list: string | object | Buffer | NodeJS.ReadableStream (this is actually copied from the current fastify.d.ts types HTTPInjectOptions so I believe it should be safe to implement here as well). Depending on what happens here I'll most likely be removing the generic.
• Am I correct that Response._lightMyRequest.payloadChunks is of type Array<Buffer>?

Not ready to be merged yet as I'm still playing with tsd to test the new types. It is a young module with a tiny API so it might not be quite ready yet, but I'd like to revisit it with a clear head tomorrow.

Finally, I need to experiment with the declaration file exports, namespace, and some of the internal TSC stuff I used (such as interface) to make sure it is correct.