Update ui-grid-column-resizer.js

It was tested in chrome, FF and IE (latest versions)
17 px is the minimum to work with all the three browsers (IE is the problem, chrome and FF are looking good with extra 2 px)
There is no problem if the header is the longest value (so only if it's not an header cell i want to add 17 px).

I don't have a solution for you If you don't want to write hard coded px.