oauth2/google: fix remove content-type header from idms get requests

This is a fix on the https://github.com/googleapis/google-cloud-go/pull/9508.
The aws provider in that library is a ported dependency from here.

Change-Id: I28e1efa4fdb8292210b695a164a55060c83dae88
GitHub-Last-Rev: c425f2d3b1
GitHub-Pull-Request: golang/oauth2#711
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/570875
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Chris Smith <chrisdsmith@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

google/externalaccount/aws.go

@@ -520,7 +520,6 @@ func (cs *awsCredentialSource) getMetadataSecurityCredentials(roleName string, h
 	if err != nil {
 		return result, err
 	}
-	req.Header.Add("Content-Type", "application/json")
 
 	for name, value := range headers {
 		req.Header.Add(name, value)