Provide a public API like RequestBody.Empty

[MukjepScarlet]

I'm using version 5.0.0-alpha14, and found that we have a public commonEmptyRequestBody in -UtilCommon.kt.

val commonEmptyHeaders: Headers = Headers.headersOf()
val commonEmptyRequestBody: RequestBody = EMPTY_BYTE_ARRAY.toRequestBody()
val commonEmptyResponse: ResponseBody = EMPTY_BYTE_ARRAY.toResponseBody()

I think we can have them internal and public them like this:

// with @JvmField

Headers.Empty
RequestBody.Empty
ResponseBody.Empty

This type might fit Kotlin more. Also we can easier access them in Java like Headers.Companion.Empty