CB-6313 [wp8]: Extra boundary in upload

When performing an upload on Windows Phone, an extra boundary is
included after any parameters, and before the file contents. This causes
some servers (such as express/multiparty on node.js) to fail to handle
the request properly.

This commit removes the extraneous boundary.

Fixes https://issues.apache.org/jira/browse/CB-6313

github close #73
diff --git a/src/wp/FileTransfer.cs b/src/wp/FileTransfer.cs
index e1f9fb0..7c29cfc 100644
--- a/src/wp/FileTransfer.cs
+++ b/src/wp/FileTransfer.cs
@@ -818,7 +818,6 @@
                             byte[] formItemBytes = System.Text.Encoding.UTF8.GetBytes(formItem);
                             requestStream.Write(formItemBytes, 0, formItemBytes.Length);
                         }
-                        requestStream.Write(boundaryBytes, 0, boundaryBytes.Length);
                     }
                     using (IsolatedStorageFile isoFile = IsolatedStorageFile.GetUserStoreForApplication())
                     {