Don't decode query string names

[timkelty]

Description

UrlHelper::buildQuery can produce invalid urls with decoded brackets ?foo[]=bar.

Because of some special cases with ?var={variables} in URLs, we need to preserve some characters from encoding, but I'm not sure why we'd ever need to decode the name. Are there any known use cases of something like:
?param[{id}]=value?

There is a history of fixes & reverts, so even though this is one line, it shouldn't be merged lightly.

I tried to dig through the history a bit:

• initial use of urldecode: 89b54ed
• moved to buildQuery: 92fa26c
• Deprecate buildQuery , and drop urldecode 2e97c56
  • This PR brings us back to this state, but without deprecating buildQuery.
• Revert fix (adds buildQuery with urldecode back): 50f50ea

I added some tests and notes where things changed.

Related issues

• Fixes [4.x]: UrlHelper::buildQuery decodes query parameter names resulting in invalid html attibutes #12796
• getSrcset produces invalid urls cloud-extension-yii2#98