Okhttp攔截器沒有看到請求的Content-type標頭

我需要創建一個OkHttp interceptor，它讀取請求的Content-Type頭的值（帶有正文的POST請求）。不幸的是，當我試圖通過request.headers訪問它時，它不在那里（我可以看到其他自定義標頭，但看不到Content-Type）。然而，我可以在HttpLoggingInterceptor的日志中清楚地看到它。

我正在使用Retrofit

Sample code:

//versions:
//okHttp: 4.10.0
//retrofit: 2.9.0

interface API {
    @Headers("secret: aaa", "aaa: AAA", "bbb: BBB", "Content-Type: application/json")
    @POST("/post")
    suspend fun echoPOST(@Body bodyData: BodyData): Response<String>
}

data class BodyData(val secret: String = "aaaa", val nonsecret: String = "bbb")

val httpClient = OkHttpClient.Builder()
            .addInterceptor(
                HttpLoggingInterceptor().setLevel(HttpLoggingInterceptor.Level.BODY)
            )
            .addInterceptor(MyInterceptor())
            .build()
val testRetrofitClient = Retrofit.Builder()
            .baseUrl("https://httpbin.org")
            .addConverterFactory(ScalarsConverterFactory.create())
            .addConverterFactory(GsonConverterFactory.create())
            .client(httpClient)
            .build()

val api = testRetrofitClient.create(API::class.java)

suspend fun testRequestPOST() {
        try {
            val response = api.echoPOST(BodyData())
        } catch (e: Exception) {
            Timber.e(e)
        }
}

interceptor:

class MyInterceptor : Interceptor {
    override fun intercept(chain: Chain): Response {
        val request = chain.request()
        val response = chain.proceed(request)
        Log.d("HTTP", "Content-Type: ${request.header("Content-Type")}")
        Log.d("HTTP", "request headers: ${request.headers.joinToString { it.first }}")
        return response
    }
}

當調用被觸發時，Logcat顯示：

// output from HttpLoggingInterceptor
--> POST https://httpbin.org/post
Content-Type: application/json; charset=UTF-8
Content-Length: 35
secret: aaa
bbb: bbb123
ccc: ccc123
{"nonsecret":"bbb","secret":"aaaa"}

// output from MyInterceptor
Content-Type: null
request headers: secret, bbb, ccc

因此Content-Type標頭存在于請求中（由HttpLoggingInterceptor記錄），但在MyInterceptor中不可見

我做錯了什么？