brew: downcase arg before sending to Homebrew
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
claui/brew,DomT4/brew,aw1621107/brew,ilovezfs/brew,mistydemeo/homebrew,gordonmcshane/homebrew,AnastasiaSulyagina/brew,staticfloat/brew,mistydemeo/homebrew,toonetown/homebrew,MikeMcQuaid/brew,toonetown/homebrew,maxim-belkin/brew,mgrimes/brew,reitermarkus/brew,pseudocody/brew,gregory-nisbet/brew,Homebrew/brew,muellermartin/dist,jmsundar/brew,gordonmcshane/homebrew,Homebrew/brew,sjackman/homebrew,tonyg/homebrew,konqui/brew,bfontaine/brew,palxex/brew,rwhogg/brew,JCount/brew,JCount/brew,reelsense/brew,hanxue/linuxbrew,hanxue/linuxbrew,mgrimes/brew,staticfloat/brew,reitermarkus/brew,DomT4/brew,muellermartin/dist,maxim-belkin/brew,rwhogg/brew,zmwangx/brew,pseudocody/brew,EricFromCanada/brew,amar-laksh/brew_sudo,konqui/brew,staticfloat/brew,Linuxbrew/brew,MikeMcQuaid/brew,nandub/brew,pseudocody/brew,reelsense/brew,toonetown/homebrew,gregory-nisbet/brew,JCount/brew,jmsundar/brew,claui/brew,alyssais/brew,tonyg/homebrew,alyssais/brew,hanxue/linuxbrew,amar-laksh/brew_sudo,alyssais/brew,bfontaine/brew,ilovezfs/brew,sjackman/homebrew,AnastasiaSulyagina/brew,Linuxbrew/brew,EricFromCanada/brew,ilovezfs/brew,JCount/brew,palxex/brew,Homebrew/brew,gregory-nisbet/brew,aw1621107/brew,reitermarkus/brew,reelsense/brew,rwhogg/brew,aw1621107/brew,reitermarkus/brew,nandub/brew,konqui/brew,Linuxbrew/brew,zmwangx/brew,Homebrew/brew,vitorgalvao/brew,gordonmcshane/homebrew,tonyg/homebrew,mistydemeo/homebrew,DomT4/brew,mgrimes/brew,AnastasiaSulyagina/brew,mahori/brew,bfontaine/brew,Linuxbrew/brew,jmsundar/brew,amar-laksh/brew_sudo,DomT4/brew,claui/brew,nandub/brew,zmwangx/brew,maxim-belkin/brew,nandub/brew,EricFromCanada/brew,palxex/brew,mahori/brew,konqui/brew,sjackman/homebrew,muellermartin/dist,claui/brew,vitorgalvao/brew,EricFromCanada/brew,mahori/brew,vitorgalvao/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,mahori/brew,sjackman/homebrew,vitorgalvao/brew