Union[str, Decimal] annotation always convert Decimal to str

[zen-xu]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.7.2
            pydantic compiled: True
                 install path: /home/xzy/dev/airport/.venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.5 (default, Aug 18 2020, 10:14:10)  [GCC 7.4.0]
                     platform: Linux-4.19.104-microsoft-standard-x86_64-with-glibc2.27
     optional deps. installed: []

from pydantic import BaseModel
from decimal import Decimal
from typing import Union

class Model(BaseModel):
    v: Union[str, Decimal]

model = Model(v=Decimal("1.1"))
model.v   # '1.1' rather than Decimal('1.1')

# But change Union position is correct
class Model(BaseModel):
    v: Union[Decimal, str]

model = Model(v=Decimal("1.1"))
model.v # Decimal('1.1')

[PrettyWood]

Hi @zen-xu
This is a know issue (see #453 or #1423)
I recently opened a PR #2092 and am planning to work on this on my spare time.
Hope it helps