Workaround for IE dropping the leading slash in pathnames

Various versions of Internet Explorer seems to neglect the leading slash under some conditions (not when watching it with the dev tools of course!). This was confirmed in IE10 + IE11

Fixes #809.
I'm not sure how this is would fix the described problem?

The problem is that IE reports the url "https://github.com/kogmbh/WebODF/pull/810" (for example) as``"kogmbh/WebODF/pull/810"`` occasionally (unless I watch it in a debugger of course).