oops, this should be added too

fuel/src/jvmTest/kotlin/fuel/HttpLoaderFactoryTest.kt

@@ -0,0 +1,20 @@
+package fuel
+
+import org.junit.Assert.assertFalse
+import java.util.concurrent.atomic.AtomicBoolean
+import kotlin.test.Test
+
+class HttpLoaderFactoryTest {
+    @Test
+    fun test_checkHttpLoaderInvokedOnce() {
+        val httpLoader = FuelBuilder().build()
+
+        val isInitialized = AtomicBoolean(false)
+        Fuel.setHttpLoader {
+            check(!isInitialized.getAndSet(true)) { "newHttpLoader was invoked more than once." }
+            httpLoader
+        }
+
+        assertFalse(isInitialized.get())
+    }
+}