Fix masgn with nested mlhs unparsing

Extracted from mbj#387

Author: viralpraxis

lib/unparser/emitter/masgn.rb

@@ -11,7 +11,13 @@ class MASGN < self
     private
 
       def dispatch
-        visit(target)
+        if target.children.one? && n_mlhs?(Util.one(target.children))
+          visit(Util.one(target.children))
+          write(',')
+        else
+          visit(target)
+        end
+
         write(' = ')
         visit(source)
       end

lib/unparser/node_helpers.rb

@@ -68,6 +68,7 @@ def n_range?(node)
       lambda
       lvar
       match_rest
+      mlhs
       or
       pair
       rescue

test/corpus/semantic/masgn.rb

@@ -0,0 +1,2 @@
+(a,), = []
+((a,),), = []