Fix example recording when using send_file

[tdeo]

When recording an example of a controller that calls send_file, I'm getting this exception because of the binary content:

Writing examples to a file
/home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/activesupport-4.2.3/lib/active_support/core_ext/object/json.rb:34:in `encode': "\xEC" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/activesupport-4.2.3/lib/active_support/core_ext/object/json.rb:34:in `to_json'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/activesupport-4.2.3/lib/active_support/core_ext/object/json.rb:34:in `to_json_with_active_support_encoder'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/activesupport-4.2.3/lib/active_support/core_ext/object/json.rb:34:in `to_json'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/activesupport-4.2.3/lib/active_support/core_ext/object/json.rb:34:in `to_json_with_active_support_encoder'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/json-1.8.3/lib/json/common.rb:285:in `generate'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/json-1.8.3/lib/json/common.rb:285:in `pretty_generate'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor/writer.rb:43:in `block in write_recorded_examples'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor/writer.rb:42:in `open'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor/writer.rb:42:in `write_recorded_examples'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor/writer.rb:13:in `write_examples'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor.rb:78:in `write_examples'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor.rb:46:in `finish'
	from /home/vagrant/pricematch/admin/vendor/ruby/2.2.0/gems/apipie-rails-0.3.4/lib/apipie/extractor.rb:179:in `block in <top (required)>'

This PR fixes it, at least for me (I'm just replacing the binary contents by a placeholder string)

[tdeo]

@iNecas Do you think this PR makes sense ?

[iNecas]

Makes sense to me. Sry I've missed this one. Merging now. Thanks @tdeo

[tdeo]

Thanks for the reactivity!

[iNecas]

Thanks for the contribution!

[iNecas]

Apipie-rails-0.5.3 with the fix was just released.