[FLINK-21974][table-api] Enable to match pattern `SET key= 'test val'…

[fsk119]

…` in SetOperationParseStrategy

What is the purpose of the change

Enable set key = 'test val' in SetOperationParseStrategy.

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Last check on commit 4bb54da (Thu Mar 25 13:03:45 UTC 2021)

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[wuchong]

I appended a commit to improve some code. Please have a look.

[fsk119]

Thanks for Jark's help. LGTM.