wrapped sub-elements serialization bug fixed

[dapper91]

This MR fixes the bug related to wrapped sub-elements serialization.

The model:

from pydantic_xml import BaseXmlModel, element, wrapped

class Company(BaseXmlModel):
    title: str = element()
    country: str = wrapped('address', element())
    city: str = wrapped('address', element())

model = Company(title='SpaceX', country='US', city='Hawthorne')

should be serialized as the xml document:

<Company>
  <title>SpaceX</title>
  <address>
    <country>US</country>
    <city>Hawthorne</city>
  </address>
</Company>

but wrapped element is duplicated:

<Company>
  <title>SpaceX</title>
  <address>
    <country>US</country>
  </address>
  <address>
    <city>Hawthorne</city>
  </address>
</Company>

fixes the issue https://github.com/dapper91/pydantic-xml/issues/63

[codecov-commenter]

Codecov Report

Merging #70 (1fdff14) into dev (a6d40e6) will increase coverage by 0.01%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##              dev      #70      +/-   ##
==========================================
+ Coverage   93.11%   93.13%   +0.01%     
==========================================
  Files          23       23              
  Lines        1089     1092       +3     
==========================================
+ Hits         1014     1017       +3     
  Misses         75       75              

Flag	Coverage Δ
unittests	93.13% <100.00%> (+0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
pydantic_xml/element/element.py	99.39% <100.00%> (+<0.01%)	:arrow_up:
pydantic_xml/serializers/factories/union.py	87.95% <100.00%> (+0.14%)	:arrow_up: