'Software' starts with capital letter as per PNG specification

Xarthisius · newville · commit 095db35fcb98 · 2016-12-27T15:52:16.000-08:00
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -558,7 +558,7 @@ def print_png(self, filename_or_obj, *args, **kwargs):
         if metadata is None:
             version_str = 'matplotlib version ' + __version__ + \
                 ', http://matplotlib.org/'
-            metadata = {six.b('software'): six.b(version_str)}
+            metadata = {six.b('Software'): six.b(version_str)}
         try:
             _png.write_png(renderer._renderer, filename_or_obj, self.figure.dpi,
                            metadata=metadata)
