Add dox comment, fix travis

python/core/auto_generated/qgsimagecache.sip.in

@@ -44,7 +44,8 @@ size, and then the result cached for subsequent lookups.
 ``path`` may be a local file, remote (HTTP) url, or a base 64 encoded string (with a "base64:" prefix).
 
 The ``size`` parameter dictates the target size of the image. An invalid size indicates the
-original raster image size (with no resampling).
+original raster image size (with no resampling). A size in which the width or height is
+set to zero will have the zeroed value automatically computed when keepAspectRatio is true.
 
 If ``keepAspectRatio`` is true, then the original raster aspect ratio will be maintained during
 any resampling operations.

src/core/qgsimagecache.h

@@ -106,7 +106,8 @@ class CORE_EXPORT QgsImageCache : public QgsAbstractContentCache< QgsImageCacheE
      * \a path may be a local file, remote (HTTP) url, or a base 64 encoded string (with a "base64:" prefix).
      *
      * The \a size parameter dictates the target size of the image. An invalid size indicates the
-     * original raster image size (with no resampling).
+     * original raster image size (with no resampling). A size in which the width or height is
+     * set to zero will have the zeroed value automatically computed when keepAspectRatio is true.
      *
      * If \a keepAspectRatio is true, then the original raster aspect ratio will be maintained during
      * any resampling operations.

tests/src/python/test_qgsimagecache.py

@@ -77,7 +77,7 @@ class TestQgsImageCache(unittest.TestCase):
         image, in_cache = QgsApplication.imageCache().pathAsImage(url, QSize(100, 100), 1.0, True)
 
         self.assertTrue(self.imageCheck('Remote Image', 'remote_image', image))
-        self.assertEqual(QgsApplication.imageCache().originalSize(url), QSize(511, 800))
+        self.assertEqual(QgsApplication.imageCache().originalSize(url), QSize(511, 800), 1.0)
 
     def testRemoteImageMissing(self):
         """Test fetching remote image with bad url"""