Fix callback behavior when no explicit url is specified in the query

[mirceanis]

What's here

This fixes #7
When using the unified request URL ( https://id.uport.me/req/... ) without an explicit callback type and url in the query, the app defaults to postback but because no postback url is set either, it does nothing with the response.

Testing

Open the example URL in the description of #7 on a mobile device.
After accepting or rejecting the call, the app should call back to https://example.com

[simonas-notcat]

Looks good

[simonas-notcat]

I take that back :) Tests are failing @mirceanis

Tests are failing

[mirceanis]

I saw.
I think the tests are wrong @simonas-notcat

For example, here the description makes sense but the test is actually matching something completely different

  ● performCallback › postback and redirect_url not set › performs returnToApp

    SagaTestError: 
    call expectation unmet:
    
    Expected
    --------
    { context: null,
      fn: [Function: postBackToServer],
      args: 
       [ { postback: undefined,
           callback_url: 'https://mydapp.com/hello?mysite=hello' },
         { status: 'ok' } ] }
    
    Actual:
    ------
    1. { context: null,
      fn: [Function: returnToApp],
      args: