Crash when using unquote_plus

[fabioz]

The following use of unquote_plus crashes IronPython (and makes it impossible to use in PyDev).

"c:\bin\IronPython 2.7.7\ipy.exe" -X:Frames -X:ExceptionDetail -X:ShowClrExceptions -c "from urllib import unquote_plus; unquote_plus('pydev%5Cplugins%5Corg')"

This worked properly on 2.7.5.

[slozier]

This was fixed in #1625.