Return back preview headers for search; remove TODOs for custom headers

[alexandear]

Recently, I found PR #2188, which suggests that we should keep custom Accept headers for preview APIs.

There's also a comment related to this:

go-github/github/github.go

Lines 59 to 74 in f066860

	// Media Type values to access preview APIs.
	// These media types will be added to the API request as headers
	// and used to enable particular features on GitHub API that are still in preview.
	// After some time, specific media types will be promoted (to a "stable" state).
	// From then on, the preview headers are not required anymore to activate the additional
	// feature on GitHub.com's API. However, this API header might still be needed for users
	// to run a GitHub Enterprise Server on-premise.
	// It's not uncommon for GitHub Enterprise Server customers to run older versions which
	// would probably rely on the preview headers for some time.
	// While the header promotion is going out for GitHub.com, it may be some time before it
	// even arrives in GitHub Enterprise Server.
	// We keep those preview headers around to avoid breaking older GitHub Enterprise Server
	// versions. Additionally, non-functional (preview) headers don't create any side effects
	// on GitHub Cloud version.
	//
	// See https://github.com/google/go-github/pull/2125 and https://github.com/google/go-github/pull/2188 for full context.

Therefore, this PR reverts my recent change in PR #3820 (commit 3959138) and removes misleading TODO comments suggesting the removal of custom Accept headers.

[gmlewis]

Ah yes, I'm very sorry for forgetting about that. My bad. Thank you, @alexandear!

[gmlewis]

LGTM.
Merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.33%. Comparing base (8657c38) to head (f066860).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3824   +/-   ##
=======================================
  Coverage   92.32%   92.33%           
=======================================
  Files         194      194           
  Lines       13979    13990   +11     
=======================================
+ Hits        12906    12917   +11     
  Misses        884      884           
  Partials      189      189           

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