Fix nil issue of example_uri_values(contract)

I got following error when simulate the contract with url like: http://abc.com?a=3&d=4

     Failure/Error: result = contracts.simulate_consumers
     TypeError:
       no implicit conversion of nil into Hash
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/actors/from_examples.rb:35:in `merge!'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/actors/from_examples.rb:35:in `build_request'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/consumer.rb:70:in `build_request'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/consumer.rb:52:in `reenact'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/contract.rb:64:in `execute'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/contract.rb:40:in `simulate_request'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/contract_set.rb:9:in `map'
     # /home/david/.rvm/gems/ruby-2.1.1/gems/pacto-0.4.0.rc1/lib/pacto/contract_set.rb:9:in `simulate_consumers'