deprecate `content_type` argument

Should we consider deprecating the `content_type` argument?
It appears redundant in conjunction with the `headers` argument. Common tools like cURL and libraries such as `requests` do not provide a specific way to set the data type; it is typically managed through headers. Therefore, I propose that we consider deprecating the `content_type` argument to streamline the function signature and simplify the codebase. Instead, we could encourage users to utilize the `headers` parameter for specifying the content type. This would enhance clarity and maintain consistency with prevailing practices.

@markstory
thoughts?