Fix exception "gsub called for nil Class" when a multipart form is su…

…bmited with a field with a nil name.

lib/www/mechanize/form.rb

@@ -216,7 +216,7 @@ def request_data
           boundary = rand_string(20)
           @enctype = "multipart/form-data; boundary=#{boundary}"
           params = []
-          query_params.each { |k,v| params << param_to_multipart(k, v) }
+          query_params.each { |k,v| params << param_to_multipart(k, v) unless k.nil? }
           @file_uploads.each { |f| params << file_to_multipart(f) }
           params.collect { |p| "--#{boundary}\r\n#{p}" }.join('') +
             "--#{boundary}--\r\n"

test/htdocs/form_test.html

@@ -145,7 +145,20 @@ <h1>Post Form 3</h1>
       </table><br />
       <input type="submit" value="Submit" />
     </form>
-    <h1>Get Form 3</h1>
+
+	<!-- Post form with multipart/form-data -->
+	<h1>Post Form 4 - Multipart</h1>
+	<form name="post_form4_multipart" enctype="multipart/form-data" method="post" action="/form_post">
+    <table>
+      <tr>
+        <td>First Name</td>
+        <td><input type="text" name="first_name" /></td>
+      </tr>
+  	</table><br />
+  	<input type="submit" value="Submit" />
+	</form>
+
+	<h1>Get Form 3</h1>
     <form name="get_form3" method="get" action="/form_post?great day=yes&one=two">
       <table>
         <tr>

test/test_forms.rb

@@ -149,6 +149,21 @@ def test_post
     )
   end
 
+  def test_post_multipart
+    page = @agent.get("http://localhost/form_test.html")
+    post_form = page.forms.find { |f| f.name == "post_form4_multipart" }
+    assert_not_nil(post_form, "Post form is null")
+    assert_equal("post", post_form.method.downcase)
+    assert_equal("/form_post", post_form.action)
+
+    assert_equal(1, post_form.fields.size)
+    assert_equal(1, post_form.buttons.size)
+
+    page = @agent.submit(post_form, post_form.buttons.first)
+
+    assert_not_nil(page)
+  end
+
   def test_select_box
     page = @agent.get("http://localhost/form_test.html")
     post_form = page.forms.find { |f| f.name == "post_form1" }