apigateway: default proxypath unless addResource(), ... are called

[rix0rrr]

Today, you must decide whether supply proxyPath: "/", or NOT supply proxyPath and call addResource().

For maximum usability, we can default to proxyPath: / but reset it to undefined once one of the addXxx() methods are called.

Also, if "/" is used for the proxy path, we should automatically install an ANY method on the root resource:

api.root.addMethod('ANY');

Otherwise, the root will not route to the proxy.