[FIX] Escape single quotes in `to_curl` serialization

When your search query contains a single quote:

    s = Tire.search 'articles' do
      query do
        string "l'amour"
      end
    end

the `to_curl` command produces an invalid JSON payload:

    curl -X GET 'http://localhost:9200/articles/_search?pretty' -d '{
      "query":{
        "query_string":{
          "query":"l'amour"
        }
      }}'

With this patch, the single quote in the query is replaced with a Unicode escape sequence:

curl -X GET 'http://localhost:9200/articles/_search?pretty' -d '{
  "query":{
    "query_string":{
      "query":"l\u0027amour"
    }
  }}'

Closes #561