allowed for multiple challenge answers (backward compatible if only 1 challenge answer passed in).

fixed request header bug in challenges

tested the following cases:

#tfa_direct - no challenge necessary
#tfa_text - {:challenges=>{:challenge=>{:text=>"Enter your first pet's name:"}}}
#tfa_text2 - {:challenges=>{:challenge=>[{:text=>"Enter your first pet's name:"}, {:text=>"Enter your high school's name:"}]}}
#tfa_choice - :challenges=>{:challenge=>{:text=>"Which high school did you attend?", :choice=>[{:text=>"Washington", :val=>"Washington"}, {:text=>"Jefferson", :val=>"Jefferson"}, {:text=>"Wilson", :val=>"Wilson"}, {:text=>"loop", :val=>"loop"}, {:text=>"Fail", :val=>"fail"}]}}}
#tfa_image - {:challenges=>{:challenge=>{:text=>"Enter the word in the dynamic image below", :image=>"/9j/4AAQSkZJRgABAQEAYABgA..."}}}
#tfa_dynamic_image - {:challenges=>{:challenge=>{:text=>"Enter the word in the dynamic image below", :image=>"/9j/4AAQS..."}}}