Fix CASE statement WHEN spacing

[rhino1998]

This fixes the CaseExpr String() method to output

CASE WHEN cond1 THEN val1 WHEN cond2 THEN val2`

instead of

CASE WHEN cond1 THEN val1WHEN cond2 THEN val2`

(notice the missing space between val1 and WHEN)

[coveralls]

Pull Request Test Coverage Report for Build 21262661987

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 51.357%

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Totals
Change from base Build 21249588693:	0.006%
Covered Lines:	8024
Relevant Lines:	15624

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💛 - Coveralls

[git-hulk]

@rhino1998 Thank you!