Pipeline Graph: Update header capitalization

What does this MR do?

The header capitalization is causing etag caching not to work (see Slack conversation: https://gitlab.slack.com/archives/C0SFP840G/p1614936464217000?thread_ts=1614874122.192600&cid=C0SFP840G [internal]). Let's fix that!

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Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #276949 (closed)