feat: add operationName to payload if defined in gql

[mydea]

We use the operation name for tracking purposes on our API. However, as already discussed here: #64, the operationName is currently not send.

In contrast to issue #64, which is about allowing to define an operationName when sending multiple queries at once, this is about sending the operationName field in the payload sent to the API for "regular" queries/mutations like this:

query allUsers {
  users
}

Where I want the server to use allUsers for tracking purposes. When we send the operationName field in our payload, like e.g. Apollo or some other GraphQL clients do, it can easily be used by GraphQL server implementations. This should be a completely backwards compatible change.

I tried to build this in a completely backwards-compatible way by extracting the actual fetch call from rawRequest into a private makeRequest function, which is called by both rawRequest and request. This way, we can pass in the (optional) operationName extracted from the query, if possible, and use that, without changing the signature of any public method.
This will basically only do anything if sending a single operation with an operation name present. Otherwise, nothing will change. If not passing a DocumentNode as query, but just a string, it will also do nothing.

I also moved the header-normalization out from the post/get methods to the request methods, to keep the get/post methods as minimal as possible.

[jasonkuhrt]

Changing the signature like this is a breaking change.

Add an overload instead for now.

[mydea]

Hmm but isn't this method private only? It is not exported, so I thought it would be easier to change it that way.
I can also add it as optional last parameter, though, if you prefer!

[jasonkuhrt]

Oh if its not part of the API then disregard this comment.

[jasonkuhrt]

LGTM