clients.defaultClient is set when no defaultClient is defined

[JohannesLamberts]

this.clients.defaultClient is set to undefined, if no defaultClient is provided:
https://github.com/Akryum/vue-apollo/blob/768e4688b98c1f517b44637d636d0babfe4d64cc/src/apollo-provider.js#L7

This leeds vue-apollo to throw on getStates in
https://github.com/Akryum/vue-apollo/blob/2f1aa2534133530e7426615b3202b81fc9f001d1/ssr/index.js#L7

IMHO one should not be forced to define the defaultClient is multiple clients are used (first for explicity, second because getStates would otherwise export redundant data that would be sent to the client).

this.clients.defaultClient and this.defaultClient should only be set if Object.hasOwnProperty(options, 'defaultClient')