spec: fix multipart form builder StringIO filename in 1.8

In 1.8 StringIO responds to path but returns nil, so it was using a different
filename. That wasn't harmful, but was inconsistent (and made the spec
fail in 1.8).

Change-Id: Ifdc37c47b4cab3a5834cd71e48453658c21c4083
Reviewed-on: https://gerrit.instructure.com/17724
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>

lib/multipart.rb

@@ -57,7 +57,7 @@ module Multipart
       def file_param(k, v)
         file_data = v.read rescue nil
         if file_data
-          file_path = v.respond_to?(:path) ? v.path : k.to_s
+          file_path = (v.respond_to?(:path) && v.path) || k.to_s
           FileParam.new(k, file_path, file_data)
         else
           Param.new(k,v)