Separate lexer and lexer_name

lib/linguist/lexer.rb

@@ -10,7 +10,7 @@ module Linguist
     end
 
     def self.find_by_alias(name)
-      @alias_index[name.downcase]
+      @alias_index[name]
     end
 
     def self.[](name)
@@ -32,7 +32,7 @@ module Linguist
     YAML.load_file(File.expand_path("../lexers.yml", __FILE__)).each do |lexer|
       @name_index[lexer.name.downcase] = lexer
       lexer.aliases.each do |name|
-        @alias_index[name.downcase] = lexer
+        @alias_index[name] = lexer
       end
     end
   end