Optimize import removes reference to invoke extension function. Add test.

#KT-3398 Fixed

idea/testData/editor/optimizeImports/multifile/InvokeFunction/InvokeFunction_after.kt

@@ -0,0 +1,7 @@
+import test1.MyClass
+import test1.invoke
+
+fun foo() {
+    val s = MyClass()
+    s()
+}

idea/testData/editor/optimizeImports/multifile/InvokeFunction/main.kt

@@ -0,0 +1,7 @@
+import test1.MyClass
+import test1.invoke
+
+fun foo() {
+    val s = MyClass()
+    s()
+}

idea/testData/editor/optimizeImports/multifile/InvokeFunction/myClass.kt

@@ -0,0 +1,6 @@
+package test1
+
+public class MyClass {
+}
+
+public fun MyClass.invoke() {}

idea/tests/org/jetbrains/jet/plugin/importOptimizer/OptimizeImportsMultiFileTest.java

@@ -38,6 +38,10 @@ public class OptimizeImportsMultiFileTest extends CodeInsightTestCase {
         doTest(getTestName(false) + "/main.kt", getTestName(false) + "/myClass.kt");
     }
 
+    public void testInvokeFunction() throws Exception {
+        doTest(getTestName(false) + "/main.kt", getTestName(false) + "/myClass.kt");
+    }
+
     public void doTest(String... fileNames) throws Exception {
         configureByFiles(null, fileNames);
         invokeFormatFile();