jinja: correct member access rule

[ngxson]

Bug spotted by @pwilkin

If object contains a key that shadow the builtin function name, then depending on syntax:

• obj.member --> the builtin takes precedence over the defined value
• obj["member"] --> the defined value takes precedence over the builtin

Example:

{{ obj.keys()|join(',') }} vs {{ obj['keys'] }} vs {{ obj.test }}

With input:

{
  "obj": {
    "keys": "value",
    "test": "attr_value"
  }
}

Results in:

keys,test vs value vs attr_value