ArgumentError: invalid byte sequence in UTF-8

It would be get error when evaluate variables has broken byte.

#### reproduce code

```
test_emit9_invalid_byte(RewriteTagFilterOutputTest):
ArgumentError: invalid byte sequence in UTF-8
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:60:in `match'
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:60:in `block in rewrite_tag'
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:57:in `each'
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:57:in `rewrite_tag'
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:47:in `block in emit'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/event.rb:54:in `call'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/event.rb:54:in `each'
  ...snip.../fluent-plugin-rewrite-tag-filter/lib/fluent/plugin/out_rewrite_tag_filter.rb:46:in `emit'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/test/output_test.rb:44:in `emit'
  ...snip.../fluent-plugin-rewrite-tag-filter/test/plugin/test_out_rewrite_tag_filter.rb:239:in `block in test_emit9_invalid_byte'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/test/input_test.rb:80:in `call'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/test/input_test.rb:80:in `block in run'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/test/base.rb:53:in `run'
  ...snip.../fluent-plugin-rewrite-tag-filter/vendor/bundle/ruby/1.9.1/gems/fluentd-0.10.43/lib/fluent/test/input_test.rb:75:in `run'
  ...snip.../fluent-plugin-rewrite-tag-filter/test/plugin/test_out_rewrite_tag_filter.rb:237:in `test_emit9_invalid_byte'
```

It has reported by bluewhale07, thank you!
https://twitter.com/bluewhale07/status/435673677309292544

#### Related issue

https://github.com/tagomoris/fluent-plugin-parser/pull/3