Handle multiple kind of requests in GraphQL controller

* Added support for GET requests
* Added support for application/graphql content-type for POST requests
* Handling query string parsing errors in controller
* Wrote specs for all this

app/controllers/graphql_controller.rb

@@ -4,11 +4,25 @@ class GraphqlController < ApplicationController
   skip_authorization_check
 
   def query
-    # ConsulSchema.execute returns the query result in the shape of a Hash, which
-    # is sent back to the client rendered in JSON
-    render json: ConsulSchema.execute(
-      params[:query],
-      variables: params[:variables] || {}
-    )
+
+    if request.headers["CONTENT_TYPE"] == 'application/graphql'
+      query_string = request.body.string  # request.body.class => StringIO
+    else
+      query_string = params[:query]
+    end
+
+    if query_string.nil?
+      render json: { message: 'Query string not present' }, status: :bad_request
+    else
+      begin
+        response = ConsulSchema.execute(
+          query_string,
+          variables: params[:variables] || {}
+        )
+        render json: response, status: :ok
+      rescue GraphQL::ParseError
+        render json: { message: 'Query string is not valid JSON' }, status: :bad_request
+      end
+    end
   end
 end