fix --test for srv record

fix `roadwork --test` for srv record.

error sample Routefile.

```
  rrset "_imaps._tcp.netblue.jp.", "SRV" do
    ttl 86400
    resource_records(
      "10 5 993 netblue.jp"
    )
  end
```

see error message below.

```
$ roadwork -t --debug
Check DNS: _imaps._tcp.netblue.jp. SRV
/Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:85:in `block (3 levels) in test': undefined method `strip' for nil:NilClass (NoMethodError)
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:85:in `map'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:85:in `block (2 levels) in test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:81:in `each'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:81:in `any?'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:81:in `block in test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:225:in `call'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:225:in `block in test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:224:in `each'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:224:in `test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl-tester.rb:38:in `test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/dsl.rb:23:in `test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/lib/roadworker/client.rb:55:in `test'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/roadworker-0.4.8/bin/roadwork:132:in `<top (required)>'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/bin/roadwork:23:in `load'
        from /Users/d6rkaiz/.rbenv/versions/2.1.2/bin/roadwork:23:in `<main>'
```