Make Language methods more resilient to non-String input (#3752)

* Add failing test for finding with non-String input

Show the failing behaviour of find_by_alias, find_by_name, and []
when non-String input is provided.

* Return nil rather than erroring on non-String input

lib/linguist/language.rb

@@ -109,7 +109,7 @@ module Linguist
     #
     # Returns the Language or nil if none was found.
     def self.find_by_name(name)
-      return nil if name.to_s.empty?
+      return nil if !name.is_a?(String) || name.to_s.empty?
       name && (@name_index[name.downcase] || @name_index[name.split(',').first.downcase])
     end
 
@@ -124,7 +124,7 @@ module Linguist
     #
     # Returns the Language or nil if none was found.
     def self.find_by_alias(name)
-      return nil if name.to_s.empty?
+      return nil if !name.is_a?(String) || name.to_s.empty?
       name && (@alias_index[name.downcase] || @alias_index[name.split(',').first.downcase])
     end
 
@@ -214,7 +214,7 @@ module Linguist
     #
     # Returns the Language or nil if none was found.
     def self.[](name)
-      return nil if name.to_s.empty?
+      return nil if !name.is_a?(String) || name.to_s.empty?
 
       lang = @index[name.downcase]
       return lang if lang

test/test_language.rb

@@ -270,6 +270,12 @@ class TestLanguage < Minitest::Test
     assert_nil Language[""]
   end
 
+  def test_does_not_blow_up_with_non_string_lookup
+    assert_nil Language.find_by_alias(true)
+    assert_nil Language.find_by_name(true)
+    assert_nil Language[true]
+  end
+
   def test_name
     assert_equal 'Perl',   Language['Perl'].name
     assert_equal 'Python', Language['Python'].name