Unset GEM_PATH with nil not empty string.

This should fix #4592, the tests all pass, but the line of code in
question goes back to 2010, so this sorta seems slightly dangerous, but
it's probable the circumstances of hitting this line in conjunction with
`bundle exec` is a combination that didn't exist prior to 1.12.x.

Issue #4592 has a full diagnosis, but the gist of it is this: if an
empty string is passed as the `GEM_PATH` to the subsequent process
launched by `bundle exec`, then if the `cmd` portion of `bundle exec` is
a ruby shebanged file, then if the current bundle install uses a local
path (`disable_shared_gems` is true) then it won't be able to find the
bundler gem at all because Bundler doesn't install itself into its own
Bundle, it's only installed in the system gems for the Ruby.

`nil` must be passed because the RubyGems code that sets up the
`GEM_PATH` does a conditional on the current `GEM_PATH` and empty string
evaluates to true, whereas `nil` evaluates to false. In the false case
the `GEM_PATH` is internally populated with the system gems path such
that the bundler gem can be found.

lib/bundler.rb

@@ -408,17 +408,20 @@ def eval_gemspec(path, contents)
     end
 
     def configure_gem_home_and_path
-      blank_home = ENV["GEM_HOME"].nil? || ENV["GEM_HOME"].empty?
+      configure_gem_path
+      configure_gem_home
+      bundle_path
+    end
+
+    def configure_gem_path(env = ENV, settings = self.settings)
+      blank_home = env["GEM_HOME"].nil? || env["GEM_HOME"].empty?
       if settings[:disable_shared_gems]
-        ENV["GEM_PATH"] = ""
+        env["GEM_PATH"] = nil
       elsif blank_home || Bundler.rubygems.gem_dir != bundle_path.to_s
         possibles = [Bundler.rubygems.gem_dir, Bundler.rubygems.gem_path]
         paths = possibles.flatten.compact.uniq.reject(&:empty?)
-        ENV["GEM_PATH"] = paths.join(File::PATH_SEPARATOR)
+        env["GEM_PATH"] = paths.join(File::PATH_SEPARATOR)
       end
-
-      configure_gem_home
-      bundle_path
     end
 
     def configure_gem_home

spec/bundler/bundler_spec.rb

@@ -140,4 +140,16 @@
       it_behaves_like "it returns the correct executable"
     end
   end
+
+  describe "configuration" do
+    context "disable_shared_gems" do
+      it "should unset GEM_PATH with nil" do
+        env = {}
+        settings = { :disable_shared_gems => true }
+        Bundler.send(:configure_gem_path, env, settings)
+        expect(env.keys).to include("GEM_PATH")
+        expect(env["GEM_PATH"]).to be_nil
+      end
+    end
+  end
 end