Fix select priority name

[vir-mir]

Now there is a problem if we want to redefine the field for the database and to work in python.

We have a table, example:

CREATE TABLE sa_tbl5 ("ID" VARCHAR(255) NOT NULL, "Name" VARCHAR(255), PRIMARY KEY ("ID"));

I initialize fields in this way:

meta = sa.MetaData()
tbl = sa.Table(
    'sa_tbl5', meta,
    sa.Column('ID', sa.String, primary_key=True, key='id'),
    sa.Column('Name', sa.String(255), key='name'),
)

Then I insert, with keys id and name, no errors, the record appeared in the database

await connect.execute(tbl.insert().values(id='test_id', name='test_name'))

Problems start when sampling data:

row = await (await connect.execute(tbl.select())).fetchone()
print(dict(row))  # {'ID': 'test_id', 'Name': 'test_name'}
print(row.name)  # AttributeError: Could not locate column in row for column 'name'

The problem is in the broken ResultMetaData, namely in the processing of result_proxy._result_map and _key_fallback.

We have to select a column from the result_proxy._result_map dictionary, and process the key attribute in it stores the current name for working in python or well as alias label.

@asvetlov @jettify will review?

[codecov]

Codecov Report

Merging #525 into master will increase coverage by 0.31%.
The diff coverage is 98.14%.

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   93.76%   94.08%   +0.31%     
==========================================
  Files          25       26       +1     
  Lines        3756     3804      +48     
  Branches      199      198       -1     
==========================================
+ Hits         3522     3579      +57     
+ Misses        196      186      -10     
- Partials       38       39       +1

Impacted Files	Coverage Δ
tests/pep492/test_sa_priority_name.py	100% <100%> (ø)
aiopg/sa/result.py	91.42% <92.85%> (+0.33%)	⬆️
tests/conftest.py	69.44% <0%> (+3.57%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 57b5d55...5e833fb. Read the comment docs.