Force name to be string when looking up mime type (#4545)

* Always pass name as string, even if nil

* Update comment

* Add nil filename test

lib/linguist/blob_helper.rb

@@ -26,14 +26,14 @@ def extname
       File.extname(name.to_s)
     end
 
-    # Internal: Lookup mime type for extension.
+    # Internal: Lookup mime type for filename.
     #
     # Returns a MIME::Type
     def _mime_type
       if defined? @_mime_type
         @_mime_type
       else
-        @_mime_type = MiniMime.lookup_by_filename(name)
+        @_mime_type = MiniMime.lookup_by_filename(name.to_s)
       end
     end
 

test/test_file_blob.rb

@@ -49,6 +49,9 @@ def test_mime_type
     assert_equal "application/xml", sample_blob("XML/bar.xml").mime_type
     assert_equal "audio/ogg", fixture_blob("Binary/foo.ogg").mime_type
     assert_equal "text/plain", fixture_blob("Data/README").mime_type
+    # GitHub doesn't use a filename when returning raw blobs
+    blob = Struct.new(:name) { include Linguist::BlobHelper }
+    assert_equal "text/plain", blob.new(nil).mime_type
   end
 
   def test_content_type