Fixed bug in the processing toolbox qgis:intersection tool

The qgis:intersection tool doesn't entirely reproduce the algorithm used in the original fTools plugin; with some data the plugin will attempt to write null geometries or geometries of the wrong type to the output shapefile (e.g. LINESTRING to a POLYGON shapefile). This commit corrects the issue.

I've uploaded some sample data as a gist, which exhibits the problem.

https://gist.github.com/snorfalorpagus/8426244

myshape.geojson - the first layer
grid.geojson - the second layer
output_fail.geojson - the output using the existing intersection tool
output_pass.geojson - the output using the modified version in this request