[Core] `HttpRequest.set_text_body` makes breaking change to Content-Type header

[mccoyp]

Key Vault SDKs sometimes manipulate request bodies during challenge-based authentication. Specifically, when sending unauthenticated requests to prompt a challenge, the request body is removed. After the challenge is received, the body content is repopulated with the set_text_body method: https://github.com/Azure/azure-sdk-for-python/blob/98810939274f70d1730e2447e78bdc3003465521/sdk/keyvault/azure-keyvault-keys/azure/keyvault/keys/_shared/challenge_auth_policy.py#L121

Today, this code works because KV uses non-DPG code generation; request objects are from azure.core.pipeline.transport instead of azure.core.rest. When DPG is used, requests start to fail because of a breaking change in HttpRequest.set_text_body's behavior: calling the method changes the request's Content-Type header from "application/json" to "text/plain", instead of leaving the header untouched (current behavior).

Recorded tests for azure-keyvault-keys succeed again after DPG generation if

request.http_request.headers["Content-Type"] = "application/json"

is inserted after any calls to set_text_body.