Testing: RestClient does not escape source HTTP parameter correctly

[spinscale]

This test reproduces the issue. Be aware, that this only happens, when the whole source is put into the source HTTP parameter and not sent as body, You can control this behaviour in RestClient.callApiBuilder() for testing purposes

---
"Expressions scripting test":

  - do:
      index:
        index:  test123
        type:   test
        id:     1
        body:   { age: 23 }

  - do:
      indices.refresh: {}

  - do: { search: { body: { script_fields : { my_field : { lang: expression, script: 'doc["age"].value + 19' } } } } }
  - match:  { hits.hits.0.fields.my_field: [ 42.0 ] }

The problem is, that everything gets encoded correctly, with the exception of the + sign, which gets replaced with a space upon decoding - and this makes the script compilation fail.