HTTP resolver times out with a non 80/443 port

I have an http endpoint that listens on port 4000 and an AppSync HTTP Datasource that points to it.

````
endpoint: http://ec2-ww-xx-yy-zz.eu-west-1.compute.amazonaws.com:4000
````

When executing a GraphQl request, the resolver times out when executing the HTTP request.

From cloudwatch:
````
com.amazonaws.deepdish.http.HttpRequestException: io.netty.channel.ConnectTimeoutException: connection timed out: ec2-xx-yy-zzz-ww.eu-west-1.compute.amazonaws.com/xx.yy.zz.ww:4000",
````

Changing port to `80` works as expected.

Start with the AppSync HTTP datasource configuration and reproduce the GraphQL request against the endpoint on port 4000, then compare it with port 80. Done means establishing whether AppSync can connect to the nonstandard port and recording the resulting behavior or limitation.

Written by the indexing model from the issue text.