Declare members of http package NonnullByDefault (#471)

* Add a package-info.java with NonnullByDefault annotation
* Make RequestCapturingClient return a response to satisfy the type checker

pkl-core/src/main/java/org/pkl/core/http/package-info.java

@@ -0,0 +1,4 @@
+@NonnullByDefault
+package org.pkl.core.http;
+
+import org.pkl.core.util.NonnullByDefault;

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

@@ -1,17 +1,41 @@
 package org.pkl.core.http
 
+import java.net.URI
+import java.net.http.HttpHeaders
 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(
     request: HttpRequest,
     responseBodyHandler: HttpResponse.BodyHandler<T>
-  ): HttpResponse<T>? {
+  ): HttpResponse<T> {
     this.request = request
-    return null
+    return dummyResponse(request)
   }
 
   override fun close() {}