Fix error with filenames ending with a dot (#3349)

The second negative argument to split instructs it to
preserve null fields in the returned array

lib/linguist/blob.rb

@@ -63,7 +63,7 @@ module Linguist
     #
     # Returns an Array
     def extensions
-      _, *segments = name.downcase.split(".")
+      _, *segments = name.downcase.split(".", -1)
 
       segments.map.with_index do |segment, index|
         "." + segments[index..-1].join(".")

test/test_language.rb

@@ -235,6 +235,7 @@ class TestLanguage < Minitest::Test
     assert_equal [Language['Clojure']], Language.find_by_filename('riemann.config')
     assert_equal [Language['HTML+Django']], Language.find_by_filename('index.jinja')
     assert_equal [Language['Chapel']], Language.find_by_filename('examples/hello.chpl')
+    assert_equal [], Language.find_by_filename('F.I.L.E.')
   end
 
   def test_find_by_interpreter