Updated with 3.8 pull request review comments

1. Corrected double underscore in ApiType.DELETE__OWNED_ENTITY
2. Removed unneeded entityId parameter in
makeParamsForUpdateCustomField() (ServiceHelper)
3. Reimplemented makeParamsForUpdateCustomField() making JSON string
using Gson.toJson() for CustomFieldValue object (instead of building
the JSON string by string concatenation.)