oauth2: handle nil headers in Transport.RoundTrip

[dims]

Fixes an issue where setting auth headers on requests with nil Headers
would cause a panic.

This change ensures that Transport.RoundTrip properly handles HTTP
requests with nil headers by initializing the header map before setting
the authorization header. Added a test case to verify this behavior as well.

The bug was introduced in 696f7b3

cc @seankhliao (original author for review please)

[dims]

Found when trying to update Kubernetes to latest oauth2 version (v0.30.0):
kubernetes/kubernetes#132133

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Patch Set 1:

(1 comment)

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Message from Davanum Srinivas (dims):

Patch Set 1:

(1 comment)

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Message from Davanum Srinivas (dims):

Patch Set 1:

(1 comment)

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