Fix bundle doctor command

Previously `bundle doctor` would fail on any bundle that does not
include git gems or plugins. This is because the previously used
`Bundler.home` does not exist unless the bundle includes git gems or
plugins. For example, with `bundle config set path .bundle`, it points
to which does not exist unless this kind of gems exist in the Gemfile.

The name `Bundler.home` is really unfortunate, it should probably be
have more descriptive name, and be private. But for now I just want to
make `bundle doctor` usable.

lib/bundler/cli/doctor.rb

@@ -100,7 +100,7 @@ def check_home_permissions
       files_not_readable_or_writable = []
       files_not_rw_and_owned_by_different_user = []
       files_not_owned_by_current_user_but_still_rw = []
-      Find.find(Bundler.home.to_s).each do |f|
+      Find.find(Bundler.bundle_path.to_s).each do |f|
         if !File.writable?(f) || !File.readable?(f)
           if File.stat(f).uid != Process.uid
             files_not_rw_and_owned_by_different_user << f

spec/commands/doctor_spec.rb

@@ -22,11 +22,17 @@
     end
   end
 
+  it "succeeds on a sane installation" do
+    bundle :doctor
+
+    expect(exitstatus).to eq(0)
+  end
+
   context "when all files in home are readable/writable" do
     before(:each) do
       stat = double("stat")
       unwritable_file = double("file")
-      allow(Find).to receive(:find).with(Bundler.home.to_s) { [unwritable_file] }
+      allow(Find).to receive(:find).with(Bundler.bundle_path.to_s) { [unwritable_file] }
       allow(File).to receive(:stat).with(unwritable_file) { stat }
       allow(stat).to receive(:uid) { Process.uid }
       allow(File).to receive(:writable?).with(unwritable_file) { true }
@@ -66,7 +72,7 @@
     before(:each) do
       @stat = double("stat")
       @unwritable_file = double("file")
-      allow(Find).to receive(:find).with(Bundler.home.to_s) { [@unwritable_file] }
+      allow(Find).to receive(:find).with(Bundler.bundle_path.to_s) { [@unwritable_file] }
       allow(File).to receive(:stat).with(@unwritable_file) { @stat }
     end