add 'test-unit' gem to test.gemfile

was getting this error:

    test/proxy_test.rb:8:in `require': cannot load such file -- test/unit (LoadError)
      from test/proxy_test.rb:8:in `<main>'

(using ruby 2.2.1)

adding test-unit solved that issue

see also: rails/rails#18572

test.gemfile

@@ -2,3 +2,4 @@ source 'https://rubygems.org'
 gem 'rack'
 gem 'rest-client', '~>1.3'
 gem 'addressable', '~>2.3'
+gem 'test-unit'

test.gemfile.lock

@@ -3,9 +3,12 @@ GEM
   specs:
     addressable (2.3.4)
     mime-types (1.23)
+    power_assert (0.2.2)
     rack (1.5.2)
     rest-client (1.6.7)
       mime-types (>= 1.16)
+    test-unit (3.0.8)
+      power_assert
 
 PLATFORMS
   ruby
@@ -14,3 +17,4 @@ DEPENDENCIES
   addressable (~> 2.3)
   rack
   rest-client (~> 1.3)
+  test-unit