Use favicon if OG image URL is not specified (#52)

src/MyWebLog.Domain/SupportTypes.fs

@@ -508,9 +508,10 @@ type OpenGraphImage = {
 
     /// <summary>The <c>meta</c> properties for this image</summary>
     /// <param name="urlTransform">A function to convert relative URLs to absolute URLs</param>
+    /// <param name="fallbackImage">A relative URL to use for the image if the image property is blank</param>
     /// <returns>A sequence of key/value pairs for this OpenGraph image file</returns>
-    member this.ToProperties(urlTransform: string -> string) = seq {
-        let url = urlTransform this.Url
+    member this.ToProperties(urlTransform: string -> string, fallbackImage: string option) = seq {
+        let url = urlTransform (if this.Url = "" then defaultArg fallbackImage "" else this.Url)
         yield "og:image", url
         if url.StartsWith "https:" then yield "og:image:secure_url", url
         match this.Type with
@@ -691,10 +692,11 @@ type OpenGraphProperties = {
 
     /// <summary>The <c>meta</c> properties for this page or post</summary>
     /// <param name="urlTransform">A function to convert relative URLs to absolute URLs</param>
+    /// <param name="fallbackImage">A relative URL to use for the image if the image property is blank</param>
     /// <returns>A sequence of key/value pairs for this set of OpenGraph properties</returns>
-    member this.ToProperties urlTransform = seq {
+    member this.ToProperties urlTransform fallbackImage = seq {
         yield "og:type", string this.Type
-        yield! this.Image.ToProperties urlTransform
+        yield! this.Image.ToProperties(urlTransform, fallbackImage)
         match this.Description with Some desc -> yield "og:description", desc | None -> ()
         match this.Determiner  with Some det  -> yield "og:determiner",  det  | None -> ()
         match this.Locale      with Some loc  -> yield "og:locale",      loc  | None -> ()