Fix fallback for monomials (#316)

* Add default monomials implementation

* Fix

Author: blegat

src/monomial_vector.jl

@@ -1,5 +1,5 @@
 function monomials(v::AbstractVariable, degree, args...)
-    return monomials((v,), degree, args...)
+    return monomials(variables(v), degree, args...)
 end
 
 """