Paths are not copied over in the copy constructor of `OpenApiPaths`

[irvinesunday]

To repro:

In the OpenApiDocumentTest.cs, add the below test:

[Fact]
private void ShouldCopyDocument()
{
    var doc = new OpenApiDocument(AdvancedDocument);
    Assert.NotEmpty(doc.Paths);
    Assert.Equal(2, doc.Paths.Count);
}

Note that the above test fails on both assertions.