Encoding error with binary files about rack-test HOT 13 CLOSED

diff --git a/lib/rack/test/utils.rb b/lib/rack/test/utils.rb
index 8f37058..079aee1 100644
--- a/lib/rack/test/utils.rb
+++ b/lib/rack/test/utils.rb
@@ -129,6 +129,8 @@ module Rack
           "\"\r\n\r\n" <<
           value.to_s <<
           "\r\n"
+        buffer.force_encoding(Encoding::BINARY)
+        buffer
       end

       # Append the multipart fragment for a parameter that is a file upload to the buffer.
@@ -144,6 +146,7 @@ module Rack
           "\r\ncontent-length: " <<
           uploaded_file.size.to_s <<
           "\r\n\r\n"
+        buffer.force_encoding(Encoding::BINARY)

         # Handle old versions of Capybara::RackTest::Form::NilUploadedFile
         if uploaded_file.respond_to?(:set_encoding)
diff --git a/spec/rack/test/utils_spec.rb b/spec/rack/test/utils_spec.rb
index 7bb72ae..a787e18 100644
--- a/spec/rack/test/utils_spec.rb
+++ b/spec/rack/test/utils_spec.rb
@@ -168,6 +168,23 @@ describe 'Rack::Test::Utils.build_multipart' do
     params['foo'].must_equal %w[1 2]
   end

+  it 'builds nested multipart bodies with UTF-8 data' do
+    files = Rack::Test::UploadedFile.new(multipart_file('mb.txt'))
+    data  = Rack::Test::Utils.build_multipart('people' => [{ 'submit-name' => "\u1234", 'files' => files }], 'foo' => %w[1 2])
+
+    options = {
+      'CONTENT_TYPE' => "multipart/form-data; boundary=#{Rack::Test::MULTIPART_BOUNDARY}",
+      'CONTENT_LENGTH' => data.length.to_s,
+      :input => StringIO.new(data)
+    }
+    env = Rack::MockRequest.env_for('/', options)
+    params = Rack::Multipart.parse_multipart(env)
+    params['people'][0]['submit-name'].must_equal  "\u1234"
+    params['people'][0]['files'][:filename].must_equal 'mb.txt'
+    params['people'][0]['files'][:tempfile].read.must_equal "\u2345".b
+    params['foo'].must_equal %w[1 2]
+  end
+
   it 'builds nested multipart bodies with an array of hashes' do
     files = Rack::Test::UploadedFile.new(multipart_file('foo.txt'))
     data  = Rack::Test::Utils.build_multipart('files' => files, 'foo' => [{ 'id' => '1', 'name' => 'Dave' }, { 'id' => '2', 'name' => 'Steve' }])
diff --git a/spec/fixtures/mb.txt b/spec/fixtures/mb.txt
new file mode 100644
index 0000000..6b68062
--- /dev/null
+++ b/spec/fixtures/mb.txt
@@ -0,0 +1 @@
+<E2><8D><85>
\ No newline at end of file