serialize hashes and arrays in variables

[rcmoret]

In a rails console if I do this:

groq = "*[ _id in $uuids] { _id }"
variables = {uuids: ["15af9fc6-bbaf-4368-ab4d-62206a35f5f5", "2703512d-c1c7-49e4-9d20-c70e85cf2ec3", "46071c04-756b-4ad3-9a8e-2cad0652a24d", "7a02ec75-ba7c-4a44-927c-1a56459cda7e", "c2f69b8b-7f24-45d5-be2c-eb48c2b12901", "fe842bc1-d532-498c-be2f-466ce924b67e"]}
# then 
Sanity::Document.where(groq: groq, variables: variables)
# => 
{"error"=>
  {"description"=>
    "Unable to parse value of \"$uuids=\"[\"15af9fc6-bbaf-4368-ab4d-62206a35f5f5\", \"2703512d-c1c7-49e4-9d20-c70e85cf2ec3\", \"46071c04-756b-4ad3-9a8e-2cad0652a24d\", \"7a02ec75-ba7c-4a44-927c-1a56459cda7e\", \"c2f
69b8b-7f24-45d5-be2c-eb48c2b12901\", \"fe842bc1-d532-498c-be2f-466ce924b67e\"]\"\". Please quote string values.",
   "type"=>"httpBadRequest"}}

This fixes that issue

[dvmonroe]

ty @rcmoret

[dvmonroe]

Adding tests in #42