Avoid upstream exceptions in ItemAdapter.__repr__

[elacuesta]

Related to #30, scrapy/itemloaders#14

Replace the repr call, display the currently set fields.

Keep in mind that this only prevents the upstream AttributeError in ItemAdapter.__repr__: displaying the original wrapped item will still fail if an attribute is missing:

>>> from dataclasses import dataclass
>>> from itemadapter import ItemAdapter
>>> 
>>> @dataclass(init=False)
... class Product:
...     name: str
...     price: float
... 
>>> p = Product()
>>> a = ItemAdapter(p)
>>> a["name"] = "foo"
>>> a  # this works
<ItemAdapter for Product(name='foo')>
>>> p  # but this still breaks
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/dataclasses.py", line 357, in wrapper
    result = user_function(self)
  File "<string>", line 2, in __repr__
AttributeError: 'Product' object has no attribute 'price'

/cc @ejulio

[codecov]

Codecov Report

Merging #31 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          110       111    +1     
=========================================
+ Hits           110       111    +1     

Impacted Files	Coverage Δ
itemadapter/adapter.py	100.00% <100.00%> (ø)

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[Gallaecio]

Cool

[elacuesta]

For the record, I will open a separate PR to add a test using @dataclass(init=False)