Simplify implementation of RequestCapturingClient (#473)

pkl-core/src/test/kotlin/org/pkl/core/http/RequestCapturingClient.kt

@@ -1,41 +1,18 @@
 package org.pkl.core.http
 
-import java.net.URI
-import java.net.http.HttpHeaders
+import org.pkl.commons.test.FakeHttpResponse
 import java.net.http.HttpRequest
 import java.net.http.HttpResponse
-import java.util.*
-import javax.net.ssl.SSLSession
 
 class RequestCapturingClient : HttpClient {
   lateinit var request: HttpRequest
 
-  private fun <T: Any?> dummyResponse(request: HttpRequest) : HttpResponse<T> {
-    return object : HttpResponse<T> {
-      override fun statusCode(): Int = throw NotImplementedError()
-
-      override fun request(): HttpRequest = request
-
-      override fun previousResponse(): Optional<HttpResponse<T>> = throw NotImplementedError()
-
-      override fun headers(): HttpHeaders = throw NotImplementedError()
-
-      override fun body(): T = throw NotImplementedError()
-
-      override fun sslSession(): Optional<SSLSession> = throw NotImplementedError()
-
-      override fun uri(): URI = throw NotImplementedError()
-
-      override fun version(): java.net.http.HttpClient.Version = throw NotImplementedError()
-    }
-  }
-
-  override fun <T : Any?> send(
+  override fun <T : Any> send(
     request: HttpRequest,
     responseBodyHandler: HttpResponse.BodyHandler<T>
   ): HttpResponse<T> {
     this.request = request
-    return dummyResponse(request)
+    return FakeHttpResponse()
   }
 
   override fun close() {}