This adds `__eq__` to handle `code` in comparisons.
When comparing an ErrorDetail to a string (missing `code` there) the
ErrorDetail's `code` is ignored, but otherwise it is taken into account.
Since `str` objects are immutable, appending to existing `str` creates
in fact a new `str` instance.
Thus `ErrorDetail.detail.code` attribute is lost after `str` concatenation operation.
