Make doReturn accept null values.

Fixes #34.

Author: nhaarman

mockito-kotlin/src/main/kotlin/com/nhaarman/mockito_kotlin/Mockito.kt

@@ -62,8 +62,8 @@ fun <T> doAnswer(answer: (InvocationOnMock) -> T?): Stubber = Mockito.doAnswer {
 
 fun doCallRealMethod(): Stubber = Mockito.doCallRealMethod()!!
 fun doNothing(): Stubber = Mockito.doNothing()!!
-fun doReturn(value: Any): Stubber = Mockito.doReturn(value)!!
-fun doReturn(toBeReturned: Any, vararg toBeReturnedNext: Any): Stubber = Mockito.doReturn(toBeReturned, *toBeReturnedNext)!!
+fun doReturn(value: Any?): Stubber = Mockito.doReturn(value)!!
+fun doReturn(toBeReturned: Any?, vararg toBeReturnedNext: Any?): Stubber = Mockito.doReturn(toBeReturned, *toBeReturnedNext)!!
 fun doThrow(toBeThrown: KClass<out Throwable>): Stubber = Mockito.doThrow(toBeThrown.java)!!
 fun doThrow(vararg toBeThrown: Throwable): Stubber = Mockito.doThrow(*toBeThrown)!!
 

mockito-kotlin/src/test/kotlin/MockitoTest.kt

@@ -230,6 +230,25 @@ class MockitoTest {
         expect(mock.stringResult()).toBe("test")
     }
 
+    @Test
+    fun testDoReturnNullValue() {
+        val mock = mock<Methods>()
+
+        doReturn(null).whenever(mock).stringResult()
+
+        expect(mock.stringResult()).toBeNull()
+    }
+
+    @Test
+    fun testDoReturnNullValues() {
+        val mock = mock<Methods>()
+
+        doReturn(null, null).whenever(mock).stringResult()
+
+        expect(mock.stringResult()).toBeNull()
+        expect(mock.stringResult()).toBeNull()
+    }
+
     @Test
     fun testDoReturnValues() {
         val mock = mock<Methods>()