Schema is not loaded when using GET method

[martinek]

Schema loading fails for me unless POST method is used.

Schema loading is not using any variables so graphQLParams['variables'] = "{}" is called. That is later JSON.stringified and graphql returns error:

Variables must be provided as an Object where each property is a variable value. Perhaps look to see if an unparsed JSON string was provided.

[skevy]

Happy to take a PR to support GET requests!