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GH-15289: [Ruby] Return self when saving Table to csv (#33653)

# Rationale for this change

Change return value of Table#save when saving Table to csv files.

# What changes are included in this PR?

Change return value to self (`@ table`).

# Are these changes tested?

Add test for saving to csv.

# Are there any user-facing changes?

Return value of Table#save is undetermined.

* Closes: #15289

Authored-by: Hirokazu SUZUKI <heronshoes877@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>