respect http header case-insensitivity
kubernetes-client/javascript#2690

to maximize backward compatibility, this PR preserves header casing.

when a new header is set that is case-insensitively equal to an existing header, the old header key and value will be deleted prior to setting the new one.

setHeaderParam('key','value') should delete a value previously set by setHeaderParam('kEy','old-value')

Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion. However,
header field names MUST be converted to lowercase prior to their
encoding in HTTP/2.

https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2

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