BUG: Correct semantic error regarding cURL options in RestfulService

cURL options are numeric, and array_merge is destructive of numeric
keys. Replace array_merge calls with array union operator, with defaults
on right-hand side so that passed options override defaults.
Merge pull request #1231 from oddnoc/3.1-restfulservice-option-merge-fix

BUG: Correct semantic error regarding cURL options in RestfulService