Improve code coverage for System.Xml.Serialization

[sepidehkh]

These are some of the bigger types with low coverage in System.Xml.Serialization. It would be great if we could add more tests to reach 80-90% coverage on these:

• System.Xml.Serialization.SoapReflectionImporter 52.4%
• System.Xml.Serialization.XmlReflectionImporter 67.2%
• System.Xml.Serialization.XmlSchemaExporter 27.5%
• System.Xml.Serialization.XmlSchemaImporter 31.6%
• System.Xml.Serialization.XmlSchemas 29.4%
• System.Xml.Serialization.XmlSerializationReader 49%
• System.Xml.Serialization.XmlSerializationWriter 54.1%

Detailed coverage report:
https://ci.dot.net/job/dotnet_corefx/job/master/job/code_coverage_windows/Code_Coverage_Report/

[danmoseley]

xml serialization= area-serialization

[sepidehkh]

Please add test to hit this block as well (so that we can track this TODO)

[StephenBonikowsky]

@HongGit To work with @imcarolwang to add test coverage.