(#27) Escape PackageProperties elements

On .Net fx, the PackageProperties elements have xml special chars
escaped. On Mono, they do not get escaped. mono/mono#21227

In the process of reading the nuspec metadata, escaped chars like "<"
get converted back into what they represent, for example "<". This is
not an issue on Windows, but is it an issue on Mono.

This manually escapes xml special chars from the PackageProperties
strings. The id and version are not escaped, as they cannot contain xml
special chars in the first place.

src/Core/Authoring/PackageBuilder.cs

@@ -6,6 +6,7 @@
 using System.IO;
 using System.IO.Packaging;
 using System.Linq;
+using System.Security;
 using NuGet.Resources;
 
 namespace NuGet
@@ -302,14 +303,14 @@ public void Save(Stream stream)
                 WriteFiles(package);
 
                 // Copy the metadata properties back to the package
-                package.PackageProperties.Creator = String.Join(",", Authors);
-                package.PackageProperties.Description = Description;
+                package.PackageProperties.Creator = SecurityElement.Escape(String.Join(",", Authors));
+                package.PackageProperties.Description = SecurityElement.Escape(Description);
                 package.PackageProperties.Identifier = Id;
                 package.PackageProperties.Version = Version.ToString();
-                package.PackageProperties.Language = Language;
-                package.PackageProperties.Keywords = ((IPackageMetadata)this).Tags;
-                package.PackageProperties.Title = Title;
-                package.PackageProperties.LastModifiedBy = CreatorInfo();
+                package.PackageProperties.Language = SecurityElement.Escape(Language);
+                package.PackageProperties.Keywords = SecurityElement.Escape(((IPackageMetadata)this).Tags);
+                package.PackageProperties.Title = SecurityElement.Escape(Title);
+                package.PackageProperties.LastModifiedBy = SecurityElement.Escape(CreatorInfo());
             }
         }