allow writing custom metadata to document builder #669 (#715)

* allow writing custom metadata to document builder #669

* skip null entries

src/UglyToad.PdfPig/Writer/PdfDocumentBuilder.cs

@@ -1092,6 +1092,11 @@ internal AddedFont(Guid id, IndirectReferenceToken reference)
         /// </summary>
         public class DocumentInformationBuilder
         {
+            /// <summary>
+            /// Consumer applications can store custom metadata in the document information dictionary.
+            /// </summary>
+            public Dictionary<string, string> CustomMetadata { get; } = new Dictionary<string, string>();
+
             /// <summary>
             /// <see cref="DocumentInformation.Title"/>.
             /// </summary>
@@ -1136,6 +1141,16 @@ internal Dictionary<NameToken, IToken> ToDictionary()
             {
                 var result = new Dictionary<NameToken, IToken>();
 
+                foreach (var pair in CustomMetadata)
+                {
+                    if (pair.Key == null || pair.Value == null)
+                    {
+                        continue;
+                    }
+
+                    result[NameToken.Create(pair.Key)] = new StringToken(pair.Value);
+                }
+
                 if (Title != null)
                 {
                     result[NameToken.Title] = new StringToken(Title);