Allow customizing the default mock response

[lundberg]

Fixes #227

Swap the regular 200 status default response with e.g. a 404 status when a route not mocked ..

respx_mock = respx.mock(default=httpx.Response(404))
respx_mock.get("https://example.com/known").respond(200)

with respx_mock:
    response = httpx.get("https://example.com/known")  # mocked
    assert response.status_code == 200

    response = httpx.get("https://example.com/unknown")  # not mocked
    assert response.status_code == 404

.. or let unmocked routes result in a network errors ..

respx_mock = respx.mock(default=httpx.NetworkError)
respx_mock.get("https://example.com/known").respond(200)

with respx_mock:
    response = httpx.get("https://example.com/known")  # mocked
    assert response.status_code == 200

    response = httpx.get("https://example.com/unknown")  # not mocked, i.e. will raise

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fc8b43b) to head (10a23c8).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #315   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2941      2984   +43     
  Branches       200       205    +5     
=========================================
+ Hits          2941      2984   +43     

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