Add keywords for border types as listed in POD in Writer/Excel.pm

I noticed no matter what I had in the border sections of my template, if I had something there my XLSX output had all four thin borders on the cell.  In particular I was shooting for partial borders because I'm testing with a template generated from a fully formatted spreadsheet which uses that option heavily.

It looks like these were just forgotten and with %border filled, the code that follows works as expected.  The existing tests only look for no borders or all thin borders, and the generator handles all the border types, so I figured it was just forgotten on the flip side here.

I wanted to provide tests for partial borders and border colors and styles, but after editing t/data/Test.json I realized that you are actually sourcing from Test.xlsx and I didn't want to mangle it somehow by using LOcalc if you intend for that file to be something definitely out of Excel or something.  I can work on another PR for that if desired.