Make track_files work when configured with nil

Fixes #462.

The implementation of `track_files` was refactored recently to suppress
a warning in the underlying instance variable had not been initialized.
This had the effect of changing the behavior when trying to nullify the
default configuration of tracked files in some cases through the use of
nil.

With this change, a declaration of `track_files nil` will once again
clear out any previous configuration and essentially disable the
feature.

Reference:
  * #447
  * #462

lib/simplecov.rb

@@ -57,9 +57,9 @@ def start(profile = nil, &block)
     # their coverage to zero.
     #
     def add_not_loaded_files(result)
-      if track_files
+      if tracked_files
         result = result.dup
-        Dir[track_files].each do |file|
+        Dir[tracked_files].each do |file|
           absolute = File.expand_path(file)
 
           result[absolute] ||= [0] * File.foreach(absolute).count

lib/simplecov/configuration.rb

@@ -50,11 +50,18 @@ def coverage_path
     # or not they were explicitly required. Without this, un-required files
     # will not be present in the final report.
     #
-    def track_files(glob = nil)
-      return @track_files if defined?(@track_files) && glob.nil?
+    def track_files(glob)
       @track_files = glob
     end
 
+    #
+    # Returns the glob that will be used to include files that were not
+    # explicitly required.
+    #
+    def tracked_files
+      @track_files if defined?(@track_files)
+    end
+
     #
     # Returns the list of configured filters. Add filters using SimpleCov.add_filter.
     #

spec/configuration_spec.rb

@@ -0,0 +1,23 @@
+require "helper"
+
+describe SimpleCov::Configuration do
+  describe "#tracked_files" do
+    context "when configured" do
+      let(:glob) { "{app,lib}/**/*.rb" }
+
+      before { SimpleCov.track_files(glob) }
+
+      it "returns the configured glob" do
+        expect(SimpleCov.tracked_files).to eq glob
+      end
+
+      context "and configured again with nil" do
+        before { SimpleCov.track_files(nil) }
+
+        it "returns nil" do
+          expect(SimpleCov.tracked_files).to be_nil
+        end
+      end
+    end
+  end
+end