Intorduced a new auto-property DOSHeader to file System/Reflection/PortableExecutable/PEHeaders.cs

We've PEHeader and CoffHeader properties in class PEHeaders(namespace System.Reflection.PortableExecutable), which return the instances of their respective classes, I propose a new property by the name of DOSHeader which will return the DOS header and DOS stub
@pharring I use Notepad++ and I've changed its tab setting to using 4 spaces instead.