Fix self.headers got modified during request

[keatonLiu]

When the json parameter is set, execute_request does not ignore the previously inferred content_type, even if data is None.

This behavior is caused by the following logic:

# set content type if it isn't set
if content_type is not None and "content-type" not in self.headers:
    self.headers["Content-Type"] = content_type

The issue is that self.headers may have already been modified by a previous request. For example, if the inferred Content-Type of the previous request was application/x-www-form-urlencoded, it remains in self.headers. As a result, the current request may incorrectly reuse that Content-Type, which is unexpected when json is used.

To fix this, we should:

1. First merge/process the headers (including any user-provided Content-Type).
2. Then check whether Content-Type is already set using the CaseInsensitiveDict behavior.
3. Only if it is not set, apply the inferred content_type.

This ensures that:

• User-specified headers always take precedence.
• The inferred Content-Type is only applied when truly missing.
• A previous request’s header does not unintentionally leak into the next request.

[keatonLiu]

Could I know why is it closed?

[rawandahmad698]

Could I know why is it closed?

Please resolve the conflicts we can resolve open and merge it today

[keatonLiu]

Could I know why is it closed?

Please resolve the conflicts we can resolve open and merge it today

I have resolved the confilcts, please check