Merge pull request #640 from JuliaSymbolics/test/isequal-missing-symbolic

Test: Add `isequal` tests for `Missing` and `Symbolic` types

Author: bowenszhu

test/basics.jl

@@ -312,6 +312,9 @@ end
     @syms a b c
     @test isequal(a + b, a + b + 0.01 - 0.01)
     @test isequal(a + NaN, a + NaN)
+
+    @test !isequal(a, missing)
+    @test !isequal(missing, b)
 end
 
 @testset "subtyping" begin