New spec for validating line caps

Because I'm obsessed with code coverage.

spec/args/draw_spec.rb

@@ -22,7 +22,7 @@ describe Squib::Args::Draw do
       draw.load!(args)
       expect(draw).to have_attributes(
         join: [Cairo::LINE_JOIN_BEVEL],
-        cap: [Cairo::LINE_CAP_ROUND]
+        cap: [Cairo::LINE_JOIN_ROUND]
         )
     end
 
@@ -44,6 +44,12 @@ describe Squib::Args::Draw do
       expect(draw).to have_attributes(dash: [[900, 1200, 1500]])
     end
 
+    it 'converts line caps to Cairo constants' do
+      args = {cap: :SQUARE}
+      draw.load! args
+      expect(draw).to have_attributes( cap: [Cairo::LINE_CAP_SQUARE] )
+    end
+
     context 'custom colors' do
 
       it 'looks up custom colors in the config' do