deprecate content_type argument

[beliaev-maksim]

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?

[ThiefMaster]

Please don't. It's nice 'syntactic sugar' for what's probably most common header. It also makes moving from httpretty much more straightforward.