Update runtests.jl to test plotting missings

tests fix for JuliaPy#593

Author: deszoeke

test/runtests.jl

@@ -37,4 +37,10 @@ rgba = pycall(pycall(PyPlot.ScalarMappable, PyObject, cmap=c,
                 0.8991926182237601  0.5144175317185697   0.4079200307574009   1.0
                 0.9657054978854287  0.9672433679354094   0.9680891964628989   1.0
                 0.4085351787773935  0.6687427912341408   0.8145328719723184   1.0
-                0.0196078431372549  0.18823529411764706  0.3803921568627451   1.0 ]
+                0.0196078431372549  0.18823529411764706  0.3803921568627451   1.0 ]
+
+# test for plotting missings
+x = [missing, 1, 2, 3, 4] 
+y = [1, 2, missing, 2, 3]
+ax = plot(x, y, marker=".")
+@test isa(ax[1], PyObject)