Fix ActiveSupport::SafeBuffer#gsub bug

Escaping failed while encoding nested parameters. 

```
NoMethodError: undefined method `bytesize' for nil:NilClass
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:7:in `block in escape'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/activesupport-3.2.13/lib/active_support/core_ext/string/output_safety.rb:169:in `gsub'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/activesupport-3.2.13/lib/active_support/core_ext/string/output_safety.rb:169:in `gsub'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:6:in `escape'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:55:in `block in encode'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:44:in `call'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:44:in `block (2 levels) in encode'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:42:in `each'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:42:in `block in encode'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:64:in `call'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:64:in `block in encode'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:62:in `each'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/parameters.rb:62:in `encode'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/utils.rb:153:in `to_query'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/request/url_encoded.rb:13:in `block in call'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/request/url_encoded.rb:21:in `match_content_type'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/request/url_encoded.rb:11:in `call'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/rack_builder.rb:139:in `build_response'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/connection.rb:377:in `run_request'
	from /srv/motorlot/shared/bundle/ruby/2.0.0/gems/faraday-0.9.0/lib/faraday/connection.rb:177:in `post'
```