Use escape_path rather than escape when encoding mailto: line

Utils.escape will encode space to +. Utils.escape_path encodes space to %20. While Gmail seems to handle pluses just fine, Outlook does not, and message will have spaces through subject and body. Using escape_path should fix this for Outlook.

Tested on Windows 7 with Chrome 32 (Gmail + Outlook), FF25, and IE11.

Fixes #211