Porting razorpage perf improvements from codeplex-2119

These changes were worth 2-3% of execution time on a page that makes heavy
use of attribute writing.

Making the overloads for writing a string public should show a small
throughput increase for a typical razor page.
cr feedback
Issue #1081 - XML DataContract Formatter can fail to write when instance type != declaredType.

Adding Functional tests appropriately.
Removing XmlSerializerInputFormatter from the list of Formatters.
Introducing a functional test for XmlSerializerInputFormatter.
Issue #1072: Adding Unit Tests for GetSupportedContentTypes, CanWriteResult and for XmlFormatters with DeclaredType as object.
Adding Api Explorer