fix: mc find --metadata work without prefix

[jiuker]

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Description

fix #4790
before:

mcli find --metadata X-Amz-Meta-Testname=33 alias/bucket/

now

mcli find --metadata testname=33 alias/bucket/

Motivation and Context

How to test this PR?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here