Fix the plus decoding in ArmeriaHttpUtil

[yzfeng2020]

Motivation:

Explain why you're making this change and what problem you're trying to solve.

Modifications:

• List the modifications you've made in detail.

Result:

• Closes #. (If this resolves the issue.)
• Describe the consequences that a user will face after this PR is merged.

[trustin]

• should be decoded only when it's used as a part of a query string. /foo+bar stays same while /foo?bar+baz is decoded into /foo?bar baz.

[github-actions]

🔍 Build Scan® (commit: d0147cc)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	❌ (failure)	https://ge.armeria.dev/s/hnmcg3dnuts46
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	❌ (failure)	https://ge.armeria.dev/s/y3rcd4miyir34
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	❌ (failure)	https://ge.armeria.dev/s/hlnrvlggefss2
build-ubicloud-standard-8-jdk-17-min-java-11	❌ (failure)	https://ge.armeria.dev/s/sjvbw2jtlgnuc
build-ubicloud-standard-8-jdk-17-leak	❌ (failure)	https://ge.armeria.dev/s/bgcquk6m4efp4
build-ubicloud-standard-8-jdk-11	❌ (failure)	https://ge.armeria.dev/s/rqmhhkw2dewdc
build-macos-12-jdk-21	❌ (failure)	https://ge.armeria.dev/s/6vnnszlyqb7ec