Scan an assembly in order to create a DataContractContentSerializer

Resolves #100.
When the `expected` value requires some setup (in this example, string formatting, concatenation, etc) I usually put it in the *Arrange* phase. That might be a mistake on my part though, as I didn't look-up that in Gerard Meszaros's xUnit Test Patterns book.

`XDocument.Parse` shouldn't be part of the *Act* phase.

The test should be:

```c#
[Theory, AutoData]
public void ScanCorrectlySerializesAttributedClassInstance(
    DataContractTestEventX @event)
{
    // Arrange
    // Act
    var actual = DataContractContentSerializer.Scan(@event.GetType().Assembly);
    var result = @event.AsSerializedString(actual);
    // Assert
    var expected = XDocument.Parse(
        "<test-event-x xmlns:i=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns=\"http://grean.rocks/dc\">" +
        "  <number>" + @event.Number + "</number>" +
        "  <text>" + @event.Text + "</text>" +
        "</test-event-x>");
    Assert.Equal(expected, XDocument.Parse(result), new XNodeEqualityComparer());
}
```