Mutations should not ever use GET

[adrian-skybaker]

If the fetcher is configured to use GET, the result is that even mutations are issued as GETs.

From the closest thing to a spec in this area (https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md):

GET requests MUST NOT be used for executing mutation operations.

See https://github.com/apollographql/apollo-client/blob/287aa465d56fea7d2171533921e73b78caac26a7/src/link/http/createHttpLink.ts#L99-L104 for behaviour that ignores a request for a GET containing a mutation.

If preserving the current behaviour is desireable for backwards compatibility, perhaps a separate "preferGet" type flag could be used, which uses POST for mutation and GET for all else.

[jasonkuhrt]

Thanks @adrian-skybaker I agree let's change this.