Fix for undefined method 'encoding' for Fixnum

Fix for this error
```
~/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/cgi/util.rb:8:in `escape': undefined method `encoding' for 1362775957:Fixnum (NoMethodError)
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/util/oauth_signature.rb:59:in `paypal_encode'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/util/oauth_signature.rb:27:in `oauth_signature'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/util/oauth_signature.rb:21:in `authorization_string'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/credential/third_party/token.rb:33:in `oauth_authentication'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/credential/third_party/token.rb:26:in `properties'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/api/merchant.rb:62:in `format_request'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/api/base.rb:83:in `api_call'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-core-0.2.7/lib/paypal-sdk/core/api/base.rb:96:in `post'
	from /Users/gvannoni/.rvm/gems/ruby-2.1.1/gems/paypal-sdk-merchant-1.106.2/lib/paypal-sdk/merchant/services.rb:148:in `TransactionSearch'
	from TransactionSearch.rb:38:in `<main>'
```