Fix encoding issue when setting params in epic boards

What does this MR do?

There was an encoding flag I missed which was causing the parsing of the search param ?key=value&key=&value to fail.

Resolves #330995

Repro:

• Feature.enable(:epic_boards)
• Feature.enable(:boards_filtered_search)
• create a group label &test
• Navigate to groups -> Epic boards
• then filter the epics by label &test
• Filtering should work and should work on refresh

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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Security

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