Reset Application.formatter attribute in init_report

For our Legacy API users, StyleGuide.init_report should reset the
formatter attribute before calling Application.make_formatter().

Closes #200

docs/source/release-notes/3.0.3.rst

@@ -23,6 +23,9 @@
 - Fix issue where users were unable to ignore plugin errors that were on
   by default. (See also `GitLab#195`_)
 
+- Fix our legacy API StyleGuide's ``init_report`` method to actually override
+  the previous formatter. (See also `GitLab#200`_)
+
 
 .. links
 .. _GitLab#164:
@@ -35,5 +38,7 @@
     https://gitlab.com/pycqa/flake8/issues/193
 .. _GitLab#195:
     https://gitlab.com/pycqa/flake8/issues/195
+.. _GitLab#200:
+    https://gitlab.com/pycqa/flake8/issues/200
 .. _this Python bug report:
     https://bugs.python.org/issue27649

src/flake8/api/legacy.py

@@ -120,6 +120,7 @@ class StyleGuide(object):
         if not issubclass(reporter, formatter.BaseFormatter):
             raise ValueError("Report should be subclass of "
                              "flake8.formatter.BaseFormatter.")
+        self._application.formatter = None
         self._application.make_formatter(reporter)
         self._application.guide = None
         # NOTE(sigmavirus24): This isn't the intended use of