Throw `PlatformNotSupportedException` in `netstandard1.3`

[dougbu]

• stop trying to use DCS in the JSON and XML formatters
• if UseDataContractSerializer or !UseXmlSerializer:
  • CanReadType(...) and CanWriteType(...) always return false
  • ReadFromStreamAsync(...) and WriteToStreamAsync(...) always throw
• change default XML formatter configuration to use XmlSerializer
• adjust and disable tests in reaction
• add tests of new netstandard1.3 behavior
• nits:
  • correct namespace of SerializerConsistencyTests; was alone in previous namespace
  • s/DataContractFormatter/XmlSerializerFormatter/ to reflect actual XmlMediaTypeFormatter test actions

[dougbu]

/ping reviewers

[dougbu]

Thanks @mconnew. Unfortunately, it appears only a select few have write access in this repo 😦 That means I need another reviewer.