Add Json write Postgres tests

src/Postgres/WithProps.fs

@@ -771,8 +771,10 @@ module Json =
     /// <param name="docId">The ID of the document to retrieve</param>
     /// <param name="sqlProps">The <c>SqlProps</c> to use to execute the query</param>
     [<CompiledName "WriteById">]
-    let writeById<'TKey> tableName (writer: StreamWriter) (docId: 'TKey) sqlProps =
-        byId tableName docId sqlProps |> writer.Write
+    let writeById<'TKey> tableName (writer: StreamWriter) (docId: 'TKey) sqlProps = backgroundTask {
+        let! json = byId tableName docId sqlProps
+        do! writer.WriteAsync json
+    }
 
     /// <summary>Retrieve JSON documents matching JSON field comparisons (<c>-&gt;&gt; =</c>, etc.)</summary>
     /// <param name="tableName">The table from which documents should be retrieved (may include schema)</param>