KTOR-6946 headers in the default request are overriding new custom headers

[igloo12]

Subsystem
Client

Motivation
I need to override the default headers with custom headers

Solution
The issue is with this code block.

val defaultRequest = DefaultRequestBuilder().apply {
    headers.appendAll(this@intercept.context.headers)
    plugin.block(this)
}

The plugin call is erasing the headers. I reversed the order and added a new method to the builder to replace the ones set by the default block

 val defaultRequest = DefaultRequestBuilder().apply {
    plugin.block(this)
    headers.replaceAll(this@intercept.context.headers)
}

[marychatte]

@igloo12 Thank you for the PR! I noticed that these changes break some tests in DefaultRequestTest.kt, and I'm not sure if it's better to change the plugin's behavior. Isn't the next workaround applicable to your case?

defaultRequest {
    headers.appendIfNameAbsent(HttpHeaders.ContentType, ContentType.Application.Json.toString())
}

[igloo12]

@marychatte, I changed the PR to override only the content type. My logic is the content type set via contextType(), which hides the underlying header setting/appending logic. When a user sets contextType, the default should be overridden because there can only be one.

What do you think?

[e5l]

@marychatte, could you check?

[marychatte]

As described in the PR, the plugin DefaultRequest can override headers that the user sets. However, the user's headers should have higher priority:

defaultRequest {
    contentType(ContentType.Application.Json) // defaultRequest header
}

val response = client.get("/") {
    contentType(ContentType.Application.Xml) // users' header
}

The final ContentType should be ContentType.Application.Xml

So, we need to set the user's headers, and also because defaultRequest logic could depend on them, for example:

defaultRequest {
   headers.appendIfNameAbsent("header", "value)
 }

But there is a small problem that some Headers functions override value completely, like with function contentType(...), because they are written through headers.set(...)

Proposed solution:

1. Set and save headers after user
2. Apply DefaultRequest plugin
3. Check that the user's headers weren't overridden: we just append firstly user header values and then DefaultRequest header values
   Note: Different check for the HttpHeaders.Cookie because if it has several values, they are presented as one ,and they are combined automatically in the function cookie(...)

[marychatte]

@e5l please review the suggested changes, I described them in the comment

[e5l]

lgtm