Print commas at the end of the line

When using line breaks, print commas behind the listed items, instead
of on a new line.

Before:
```
Expected someCollection {
    item1
    ,
    item2
    ,
```
After:
```
Expected someCollection {
    item1,
    item2,
```

Src/FluentAssertions/Formatting/DefaultValueFormatter.cs

@@ -42,7 +42,7 @@ public void Format(object value, FormattedObjectGraph formattedGraph, Formatting
         {
             if (context.UseLineBreaks)
             {
-                formattedGraph.AddLine(value.ToString());
+                formattedGraph.AddFragmentOnNewLine(value.ToString());
             }
             else
             {