Fix exception when saving NuGet.config settings.

InvalidOperationException thrown by Mono 2.10.9:
    This XmlWriter does not accept Text at this state Prolog.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: This XmlWriter does not accept Text at this state Prolog.
  at System.Xml.XmlTextWriter.ShiftStateContent (System.String occured, Boolean allowAttribute)
  at System.Xml.XmlTextWriter.WriteString (System.String text)
  at System.Xml.DefaultXmlWriter.WriteString (System.String text)
  at System.Xml.Linq.XText.WriteTo (System.Xml.XmlWriter w)
  at System.Xml.Linq.XDocument.WriteTo (System.Xml.XmlWriter w)
  at System.Xml.Linq.XDocument.Save (System.Xml.XmlWriter w)
  at System.Xml.Linq.XDocument.Save (System.IO.Stream stream, SaveOptions options)
  at System.Xml.Linq.XDocument.Save (System.IO.Stream stream)
  at NuGet.PhysicalFileSystem.AddFileCore (System.String path, System.Action`1 writeToStream)
  at NuGet.PhysicalFileSystem.AddFile (System.String path, System.Action`1 writeToStream)
  at NuGet.Settings.Save ()
  at NuGet.Settings.SetValueInternal (System.Xml.Linq.XElement sectionElement, System.String key, System.String value)
  at NuGet.Settings.SetValue (System.String section, System.String key, System.String value)
  at NuGet.PackageRestoreConsent.set_IsGrantedInSettings (Boolean value)

Workaround is to load the settings without preserving whitespace.

Looks like this has been fixed in mono 3.0.12
https://bugzilla.xamarin.com/show_bug.cgi?id=3231
https://github.com/mono/mono/commit/2927d27304ae9d2ea2e40f9277055bcfde2bb554