support empty string multipart filename

Closes #702.

Enables support for IE11.

lib/rack/multipart/parser.rb

@@ -116,6 +116,10 @@ def get_current_head_and_filename_and_content_type_and_name_and_body
               name = filename
             end
 
+            if filename && filename.empty?
+              filename = name
+            end
+
             if filename
               (@env['rack.tempfiles'] ||= []) << body = Tempfile.new("RackMultipart")
               body.binmode  if body.respond_to?(:binmode)

test/multipart/ie11

@@ -0,0 +1,6 @@
+--AaB03x
+Content-Disposition: form-data; name="files"; filename=""
+Content-Type: text/plain
+
+contents
+--AaB03x--

test/spec_multipart.rb

@@ -230,12 +230,15 @@ def rd.length
     params["files"][:tempfile].read.should.equal "contents"
   end
 
-  should "not include file params if no file was selected" do
+  # n.b. this case used to be "do not include", but because ie11 now does this
+  # for selected files, that can no longer be done. It was decided that not
+  # losing data is better, and no browser is documented with this behavior.
+  should "include file params if no file was selected" do
     env = Rack::MockRequest.env_for("/", multipart_fixture(:none))
     params = Rack::Multipart.parse_multipart(env)
     params["submit-name"].should.equal "Larry"
-    params["files"].should.equal nil
-    params.keys.should.not.include "files"
+    params["files"].should.not.equal nil
+    params.keys.should.include "files"
   end
 
   should "parse multipart/mixed" do
@@ -267,6 +270,19 @@ def rd.length
     params["files"][:tempfile].read.should.equal "contents"
   end
 
+  should "parse IE11 multipart upload" do
+    env = Rack::MockRequest.env_for("/", multipart_fixture(:ie11))
+    params = Rack::Multipart.parse_multipart(env)
+    params["files"][:type].should.equal "text/plain"
+    params["files"][:filename].should.equal "files"
+    params["files"][:head].should.equal "Content-Disposition: form-data; " +
+      "name=\"files\"; " +
+      'filename=""' +
+      "\r\nContent-Type: text/plain\r\n"
+    params["files"][:name].should.equal "files"
+    params["files"][:tempfile].read.should.equal "contents"
+  end
+
   should "parse filename and modification param" do
     env = Rack::MockRequest.env_for("/", multipart_fixture(:filename_and_modification_param))
     params = Rack::Multipart.parse_multipart(env)