Fix results_dir if not specified, and update changelog

Author: astrofrog

CHANGES.md

@@ -2,7 +2,19 @@
 ----------------
 
 - Fixed use of mpl_image_compare on methods of test classes that also
-  use setup_method. [#50]
+  use setup_method. [#51]
+
+- Make it possible to specify the directory in which to put the results
+  from the tests using --mpl-results-path. [#39]
+
+- Only import Matplotlib if the plugin is actually used. [#47]
+
+- Make sure figures are closed after saving. [#46]
+
+- Allow the backend to be set on a per-test basis. [#38]
+
+- If test name contains slashes (normally from parameters in
+  parametrized tests), replace with _. [#50]
 
 0.7 (2016-11-26)
 ----------------

pytest_mpl/plugin.py

@@ -77,6 +77,10 @@ def pytest_configure(config):
         generate_dir = config.getoption("--mpl-generate-path")
         results_dir = config.getoption("--mpl-results-path") or config.getini("mpl-results-path")
 
+        # Note that results_dir is an empty string if not specified
+        if not results_dir:
+            results_dir = None
+
         if generate_dir is not None:
             if baseline_dir is not None:
                 warnings.warn("Ignoring --mpl-baseline-path since --mpl-generate-path is set")