ignore content_type from credentials provider

[VasilyStepanov]

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Yes!

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Default metadata credentials provider in yandex cloud environment (http://169.254.169.254/computeMetadata/v1/instance/service-accounts/default/token) returns access token in json format, but with text/plain content type.
This is wrong, but code from iam.auth.MetadataUrlCredentials._make_token_request() ignores content type and everything works fine in sync implementation.
On the other hand, its async alternative aio.iam.MetadataUrlCredentials._make_token_request() fails with wrong content type error.

Issue Number: N/A

What is the new behavior?

Now we ignore content type from metadata credentials provider. This is also wrong, but acceptable, since sync implementation also do this.

Other information

[rekby]

Hello, @VasilyStepanov.

Thanks for the fix :)