根据https://github.com/plataformatec/devise/wiki/OmniAuth:-Overview指南尝试执行oauth登录时，为什么会出现以下错误？
`add_camelization': undefined method `[]=' for nil:NilClass (NoMethodError)
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/omniauth-oauth2-1.1.0/lib/omniauth/strategies/oauth2.rb:117:in `<top (required)>'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.2.7/lib/active_support/dependencies.rb:251:in `require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.2.7/lib/active_support/dependencies.rb:251:in `block in require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.2.7/lib/active_support/dependencies.rb:236:in `load_dependency'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.2.7/lib/active_support/dependencies.rb:251:in `require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/oa-oauth-0.0.1/lib/omniauth/oauth.rb:8:in `<top (required)>'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:68:in `require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:68:in `block (2 levels) in require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:66:in `each'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:66:in `block in require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:55:in `each'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:55:in `require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320@global/gems/bundler-1.1.5/lib/bundler.rb:119:in `require'
from /home/jeevan/gypsy/config/application.rb:7:in `<top (required)>'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:53:in `require'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:53:in `block in <top (required)>'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:50:in `tap'
from /home/jeevan/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:50:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'