Force response_body method return String.

This is fixed when `response.body` return array will trigger exception error.

#### Error message:

    NoMethodError: undefined method `gsub!' for #<Array:0x00000004d77770>

#### Backtrace

```
/vendor/bundle/ruby/1.9.1/bundler/gems/rack-honeypot-1f4584407e10/lib/rack/honeypot.rb:68 in "insert_honeypot"
/vendor/bundle/ruby/1.9.1/bundler/gems/rack-honeypot-1f4584407e10/lib/rack/honeypot.rb:27 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-pjax-0.7.0/lib/rack/pjax.rb:12 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/best_standards_support.rb:17 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/etag.rb:23 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/conditionalget.rb:25 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/head.rb:14 in "call"
/vendor/bundle/ruby/1.9.1/gems/remotipart-1.2.1/lib/remotipart/middleware.rb:27 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/params_parser.rb:21 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/flash.rb:242 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/session/abstract/id.rb:210 in "context"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/session/abstract/id.rb:205 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/cookies.rb:341 in "call"
/vendor/bundle/ruby/1.9.1/gems/activerecord-3.2.14/lib/active_record/query_cache.rb:64 in "call"
/vendor/bundle/ruby/1.9.1/gems/activerecord-3.2.14/lib/active_record/connection_adapters/abstract/connection_pool.rb:479 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/callbacks.rb:28 in "block in call"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/callbacks.rb:405 in "_run__3936741981158055881__call__2134921171264788526__callbacks"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/callbacks.rb:405 in "__run_callback"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/callbacks.rb:385 in "_run_call_callbacks"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/callbacks.rb:81 in "run_callbacks"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/callbacks.rb:27 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/remote_ip.rb:31 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/debug_exceptions.rb:16 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/show_exceptions.rb:56 in "call"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/rack/logger.rb:32 in "call_app"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/rack/logger.rb:16 in "block in call"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/tagged_logging.rb:22 in "tagged"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/rack/logger.rb:16 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/request_id.rb:22 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/methodoverride.rb:21 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/runtime.rb:17 in "call"
/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.14/lib/active_support/cache/strategy/local_cache.rb:72 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-1.4.5/lib/rack/lock.rb:15 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-rewrite-1.3.3/lib/rack/rewrite.rb:20 in "call"
/vendor/bundle/ruby/1.9.1/gems/actionpack-3.2.14/lib/action_dispatch/middleware/static.rb:63 in "call"
/vendor/bundle/ruby/1.9.1/gems/rack-cache-1.2/lib/rack/cache/context.rb:136 in "forward"
/vendor/bundle/ruby/1.9.1/gems/rack-cache-1.2/lib/rack/cache/context.rb:245 in "fetch"
/vendor/bundle/ruby/1.9.1/gems/rack-cache-1.2/lib/rack/cache/context.rb:185 in "lookup"
/vendor/bundle/ruby/1.9.1/gems/rack-cache-1.2/lib/rack/cache/context.rb:66 in "call!"
/vendor/bundle/ruby/1.9.1/gems/rack-cache-1.2/lib/rack/cache/context.rb:51 in "call"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/engine.rb:484 in "call"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/application.rb:231 in "call"
/vendor/bundle/ruby/1.9.1/gems/railties-3.2.14/lib/rails/railtie/configurable.rb:30 in "method_missing"
/vendor/bundle/ruby/1.9.1/gems/unicorn-4.5.0/lib/unicorn/http_server.rb:552 in "process_client"
/vendor/bundle/ruby/1.9.1/gems/unicorn-4.5.0/lib/unicorn/http_server.rb:628 in "worker_loop"
/vendor/bundle/ruby/1.9.1/gems/newrelic_rpm-3.6.9.171/lib/new_relic/agent/instrumentation/unicorn_instrumentation.rb:22 in "call"
/vendor/bundle/ruby/1.9.1/gems/newrelic_rpm-3.6.9.171/lib/new_relic/agent/instrumentation/unicorn_instrumentation.rb:22 in "block (4 levels) in <top (required)>"
/vendor/bundle/ruby/1.9.1/gems/unicorn-4.5.0/lib/unicorn/http_server.rb:500 in "spawn_missing_workers"
/vendor/bundle/ruby/1.9.1/gems/unicorn-4.5.0/lib/unicorn/http_server.rb:142 in "start"
/vendor/bundle/ruby/1.9.1/gems/unicorn-4.5.0/bin/unicorn:121 in "<top (required)>"
/vendor/bundle/ruby/1.9.1/bin/unicorn:23 in "load"
/vendor/bundle/ruby/1.9.1/bin/unicorn:23 in "<main>"
```