Allow classifier to run on symlinks as usual (#3948)

* Fixups for symlink detection, incl. test

* assert the heuristics return none for symlink

lib/linguist/classifier.rb

@@ -18,8 +18,6 @@ module Linguist
     #
     # Returns an Array of Language objects, most probable first.
     def self.call(blob, possible_languages)
-      return [] if blob.symlink?
-
       language_names = possible_languages.map(&:name)
       classify(Samples.cache, blob.data[0...CLASSIFIER_CONSIDER_BYTES], language_names).map do |name, _|
         Language[name] # Return the actual Language objects

lib/linguist/file_blob.rb

@@ -27,7 +27,7 @@ module Linguist
     end
 
     def symlink?
-      return @symlink if !@symlink.nil?
+      return @symlink if defined? @symlink
       @symlink = (File.symlink?(@fullpath) rescue false)
     end