__unset_gj_flag_sub_field has sometimes raises error.

[reversetracker]

Describe the bug
A clear and concise description of what the bug is.

The document.py file has a function named "__unset_gj_flag_sub_field"
it sometimes ( not all times ) raises error even the same query usually works but very sometimes.

the exception log is as below.

File "/home/yongwoo/.conda/envs/pingme/lib/python3.6/site-packages/mongoengine_goodjson/document.py", line 155, in unset_flag
   cur_depth_attr = getattr(fld, "$$cur_depth$$")
AttributeError: 'StringField' object has no attribute '$$cur_depth$$'

So here is my fixed code and it works fine again but I never understand how come it happens.

def __unset_gj_flag_sub_field(self, name, fld, cur_depth):
    """Remove current depth to subfield."""
    # def unset_flag(fld, depth_lv):
    #     setattr(fld, "$$cur_depth$$", depth_lv - 1)
    #     cur_depth_attr = getattr(fld, "$$cur_depth$$")
    #     if (not isinstance(cur_depth_attr, int)) or cur_depth_attr < 0:
    #         delattr(fld, "$$cur_depth$$")
    
    def unset_flag(fld, depth_lv):
        __cur_depth = depth_lv - 1
        if isinstance(__cur_depth, int) and __cur_depth >= 0:
            setattr(fld, "$$cur_depth$$", __cur_depth)

    self.__apply_element(
        name, fld, cur_depth, unset_flag, "end_goodjson"
    )

because you declared setattr(fld, "$$cur_depth$$", depth_lv - 1) so that getattr(fld, "$$cur_depth$$") must always have $$cur_depth$$ property but as you see the error log above it raises..

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Expected behavior
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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[reversetracker]

if you find out why it happens, please let me know