Update raven.js to work around IE8 shortcomings

Internet Explorer 8 chokes on an undefined or null reference passed into this function. Wrapping an if statement around the processing correctly handles this while still performing correctly.
Test whether joinRegExp processes empty or undefined variables

Raven throws an error if empty or undefined variables are processed by joinRegExp. Some browsers (such as Internet Explorer 8 and possibly earlier) apparently pass these variables, causing errors.
Use the isUndefined helper rather than typeof in joinRegExp
Merge pull request #179 from johnbacon/patch-1

Update raven.js to work around IE8 shortcomings