Flatten Pod::Block contents array

Due to new GLR changes, the contents attribute is handed an Array that
doesn't want to flatten.

src/core/Pod.pm

@@ -7,6 +7,12 @@ my class Pod::Block {
             DEPRECATED("Pod::Block.new(:contents)", |<2014.09 2015.09>, :up(5), :what('Pod::Block.new(:content)'));
             @!contents.push: $content.list;
         }
+        my @new-contents;
+
+        for @!contents {
+            @new-contents.push: flat @($_);
+        }
+        @!contents = @new-contents;
     }
 
     sub pod-gist(Pod::Block $pod, $level = 0) {