fix: Remove content-type header from IDMS GET requests

[peter-volkov]

Detailed description is here
https://issuetracker.google.com/issues/328089077

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[parthea]

See googleapis/google-cloud-go#9508 (comment)

[BigTailWolf]

LGTM

[BigTailWolf]

See googleapis/google-cloud-go#9508 (comment)

Hi Anthonios, the golang/oauth2 library is merged. And seems this fix is also good to go.

[chalmerlowe]

@peter-volkov thanks for contributing this. I appreciate it. Apologies that it has languished for so long. As one of the maintainers I am engaged in a project to clear out all the PRs in google-auth-library-python (so that we can migrate this repo into our google-cloud-python mono-repository). I am down to the last few and your's is one of them.

If you happen to be around, are still interested in this PR, and have a few moments, I am working on merging this. It does not have a CLA signature from you. See the second comment for some more details and a link that will take you to a signature opportunity.

In the interest of keeping things moving, if I don't have a response from you by Monday Jan 12 @ Noon EST, I will close this PR and fire up one in my own name so that we can proceed with a merge. Sorry if this feels rushed, that is not my intent. It is simply the reality of how this has played out in terms of our workload and team priorities.

It looks like I have six tests that need to be updated to ensure that this passes.

=========================== short test summary info ============================
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_temp_creds_no_environment_vars

AssertionError: assert {} == {'Content-Typ...ication/json'}
  
  Right contains 1 more item:
  {'Content-Type': 'application/json'}
  
  Full diff:
  + {}
  - {
  -     'Content-Type': 'application/json',
  - }
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_temp_creds_no_environment_vars_idmsv2
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_temp_creds_environment_vars_missing_secret_access_key_idmsv2
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_temp_creds_environment_vars_missing_access_key_id_idmsv2
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_temp_creds_environment_vars_missing_creds_idmsv2
FAILED tests/test_aws.py::TestCredentials::test_retrieve_subject_token_success_ipv6

[chalmerlowe]

Closing in deference to PR #1934 as we prepare to migrate this library to the google-cloud-python monorepo.