brew: downcase arg before sending to Homebrew

Fixes a case where brew would find commands with wrongly-capitalized
user input on case-insensitive systems, then error out with a confusing
message when an invalid method was sent to Homebrew. e.g.:

> brew Update
Error: undefined method `Update' for Homebrew:Module

bin/brew

@@ -77,7 +77,7 @@ begin
   elsif require? `/usr/bin/which brew-#{cmd}.rb`
     exit 0
   elsif require? HOMEBREW_REPOSITORY/"Library/Homebrew/cmd"/cmd
-    Homebrew.send cmd.to_s.gsub('-', '_')
+    Homebrew.send cmd.to_s.gsub('-', '_').downcase
   else
     onoe "Unknown command: #{cmd}"
     exit 1