Use a named function for the middleware over an anonymous (#415)

* Use a named function for the middleware over an anonymous

It's making the investigation of performance issues easier.

* Rename function to 'graphqlMiddleware'.

src/index.js

@@ -131,7 +131,7 @@ function graphqlHTTP(options: Options): Middleware {
  throw new Error('GraphQL middleware requires options.');
  }
 
- return (request: $Request, response: $Response) => {
+ return function graphqlMiddleware(request: $Request, response: $Response) {
  // Higher scoped variables are referred to at various stages in the
  // asynchronous state machine below.
  let params;