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GH-36809: [Python] MapScalar.as_py with custom field name (#36830)

### Rationale for this change

`MapScalar.as_py` doesn't take into account custom key/value field names

### What changes are included in this PR?

Fix and tests 

### Are these changes tested?

Simple unit test

### Are there any user-facing changes?

No API changes.
* Closes: #36809

Authored-by: aandres <aandres@tradewelltech.co>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>