Add an empty constructor for SimpleTarget.

Fixes #170.

library/src/androidTest/java/com/bumptech/glide/request/target/SimpleTargetTest.java

 package com.bumptech.glide.request.target;
 
 import com.bumptech.glide.request.animation.GlideAnimation;
-
 import org.junit.Test;
 
+import static org.mockito.Mockito.mock;
+
 public class SimpleTargetTest {
 
     @Test(expected = IllegalArgumentException.class)
-    public void testThrowsIfGivenWidthIsLessThanZero() {
-        new SimpleTarget<Object>(-1, 1) {
-
-            @Override
-            public void onResourceReady(Object resource, GlideAnimation<Object> glideAnimation) {
+    public void testThrowsOnGetSizeIfGivenWidthIsLessThanZero() {
+        getTarget(-1, 1).getSize(mock(SizeReadyCallback.class));
+    }
 
-            }
-        };
+    @Test(expected = IllegalArgumentException.class)
+    public void testThrowsOnGetSizeIfGivenWidthIsEqualToZero() {
+        getTarget(0, 1).getSize(mock(SizeReadyCallback.class));
     }
 
     @Test(expected = IllegalArgumentException.class)
-    public void testThrowsIfGivenWidthIsEqualToZero() {
-        new SimpleTarget<Object>(0, 1) {
+    public void testThrowsOnGetSizeIfGivenHeightIsLessThanZero() {
+        getTarget(1, -1).getSize(mock(SizeReadyCallback.class));
+    }
 
-            @Override
-            public void onResourceReady(Object resource, GlideAnimation<Object> glideAnimation) {
+    @Test(expected = IllegalArgumentException.class)
+    public void testThrowsOnGetSizeIfGivenHeightIsEqualToZero() {
+        getTarget(1, 0).getSize(mock(SizeReadyCallback.class));
+    }
 
-            }
-        };
+    @Test
+    public void testCanBeConstructedWithoutDimensions() {
+        getTarget();
     }
 
     @Test(expected = IllegalArgumentException.class)
-    public void testThrowsIfGivenHeightIsLessThanZero() {
-        new SimpleTarget<Object>(1, -1) {
+    public void testThrowsOnGetSizeIfConstructedWithoutDimensions() {
+        getTarget().getSize(mock(SizeReadyCallback.class));
+    }
 
+    private SimpleTarget<Object> getTarget() {
+        return new SimpleTarget<Object>() {
             @Override
-            public void onResourceReady(Object resource, GlideAnimation<Object> glideAnimation) {
-
+            public void onResourceReady(Object resource, GlideAnimation<? super Object> glideAnimation) {
+                // Do nothing.
             }
         };
     }
 
-    @Test(expected = IllegalArgumentException.class)
-    public void testThrowsIfGivenHeightIsEqualToZero() {
-        new SimpleTarget<Object>(1, 0) {
-
+    private SimpleTarget<Object> getTarget(int width, int height) {
+        return new SimpleTarget<Object>(width, height) {
             @Override
-            public void onResourceReady(Object resource, GlideAnimation<Object> glideAnimation) {
-
+            public void onResourceReady(Object resource, GlideAnimation<? super Object> glideAnimation) {
+                // Do nothing.
             }
         };
     }

library/src/main/java/com/bumptech/glide/request/target/SimpleTarget.java

@@ -27,6 +27,20 @@ public abstract class SimpleTarget<Z> extends BaseTarget<Z> {
     private final int width;
     private final int height;
 
+    /**
+     * Constructor for the target that assumes you will have called
+     * {@link com.bumptech.glide.GenericRequestBuilder#override(int, int)} on the request builder this target is given
+     * to.
+     *
+     * <p>
+     *     Requests that load into this target will throw an {@link java.lang.IllegalArgumentException} if
+     *     {@link com.bumptech.glide.GenericRequestBuilder#override(int, int)} was not called on the request builder.
+     * </p>
+     */
+    public SimpleTarget() {
+        this(-1, -1);
+    }
+
     /**
      * Constructor for the target that takes the desired dimensions of the decoded and/or transformed resource.
      *
@@ -34,10 +48,6 @@ public abstract class SimpleTarget<Z> extends BaseTarget<Z> {
      * @param height The desired height of the resource.
      */
     public SimpleTarget(int width, int height) {
-        if (width <= 0 || height <= 0) {
-            throw new IllegalArgumentException("Width and height must both be > 0, but given width: " + width + " and"
-                    + " height: " + height);
-        }
         this.width = width;
         this.height = height;
     }
@@ -49,6 +59,10 @@ public abstract class SimpleTarget<Z> extends BaseTarget<Z> {
      */
     @Override
     public final void getSize(SizeReadyCallback cb) {
+        if (width <= 0 || height <= 0) {
+            throw new IllegalArgumentException("Width and height must both be > 0, but given width: " + width + " and"
+                    + " height: " + height + ", either provide dimensions in the constructor or call override()");
+        }
         cb.onSizeReady(width, height);
     }
 }