Fix RDF.PrefixMap.merge/3 when the merged prefixes are not a PrefixMap

lib/rdf/prefix_map.ex

@@ -138,6 +138,8 @@ defmodule RDF.PrefixMap do
   If everything could be merged, an `:ok` tuple is returned.
 
   """
+  def merge(prefix_map1, prefix_map2, fun)
+
   def merge(%__MODULE__{map: map1}, %__MODULE__{map: map2}, fun) when is_function(fun) do
     conflict_resolution = fn prefix, namespace1, namespace2 ->
       case fun.(prefix, namespace1, namespace2) do
@@ -154,6 +156,10 @@ defmodule RDF.PrefixMap do
     end
   end
 
+  def merge(%__MODULE__{} = prefix_map1, prefix_map2, fun) when is_function(fun) do
+    merge(prefix_map1, new(prefix_map2), fun)
+  end
+
   def merge(prefix_map1, prefix_map2, nil), do: merge(prefix_map1, prefix_map2)
 
   defp resolved_merge_rest_conflicts(map) do

test/unit/prefix_map_test.exs

@@ -142,6 +142,9 @@ defmodule RDF.PrefixMapTest do
       other_prefix_map = PrefixMap.new(ex3: @ex_ns4)
       assert PrefixMap.merge(@example3, other_prefix_map,
                 fn _prefix, ns1, _ns2 -> ns1 end) == {:ok, @example3}
+
+      assert PrefixMap.merge(@example1, %{ex1: EX},
+               fn _prefix, _ns1, ns2 -> ns2 end) == {:ok, PrefixMap.new(ex1: EX)}
     end
 
     test "with a function which does not resolve by returning nil" do