send_request should not update _default_headers

[tanoda]

HTTPClient.send_request updates its _default_headers with headers from its parameter.
I don't believe this is the thing that you want.

[tanoda]

In addition, _default_headers shares its dict instance with the RequestExecutor.

okta-sdk-python/okta/http_client.py

Line 24 in 597908a

self._default_headers = http_config["headers"]

okta-sdk-python/okta/request_executor.py

Line 70 in 597908a

'headers': self._default_headers,

So RequestExecutor.create_request also adds the latest headers.

okta-sdk-python/okta/request_executor.py

Line 123 in 597908a

headers = {**self._default_headers, **headers}