GraphQL supports supplying multiple queries in the `query` parameter,
but an additional `operationName` parameter is then required to select
the query to execute.
Previously, it was impossible to pass `operationName` since it would get
serialized under `variables`, but it needs to be a top-level parameter.
With this change, `operationName` is a special GraphQL parameter name
just like `query` already is.
Reading from file via `-F foo=@myfile.txt` syntax would result in
`[]byte` Go type, which by default gets serialized to JSON in base64
format, which we don't want here.
Traverse all parameters and convert any `[]byte` into `string` before
JSON serialization.
