Revised fix to make browser console show hyperlinks in stack traces

Changed according to @dgreensp recommendations at https://github.com/meteor/meteor/pull/732#issuecomment-13975991:

How about this: If all the arguments are strings, then join them on " " to make a single string argument. Otherwise, leave them alone. This will fix cases like Meteor._debug("Error:", e.stack) without otherwise changing the behavior in any case that I'm aware of.