Fix hashes that require lvar omissions

[fix #367]

lib/unparser/emitter/pair.rb

@@ -15,19 +15,30 @@ class Pair < self
     private
 
       def dispatch
-        if colon?(key)
-          write(key.children.first.to_s, ': ')
+        if colon?
+          emit_colon
+          unless implicit_value?
+            write(' ')
+            visit(value)
+          end
         else
           visit(key)
           write(' => ')
+          visit(value)
         end
-
-        visit(value)
       end
 
-      def colon?(key)
+      def colon?
         n_sym?(key) && BAREWORD.match?(key.children.first)
       end
+
+      def emit_colon
+        write(key.children.first.to_s, ':')
+      end
+
+      def implicit_value?
+        n_lvar?(value) && value.children.first.equal?(key.children.first)
+      end
     end
   end
 end

lib/unparser/node_helpers.rb

@@ -52,6 +52,7 @@ def n?(type, node)
       kwargs
       kwsplat
       lambda
+      lvar
       match_rest
       pair
       rescue

test/corpus/literal/def.rb

@@ -132,3 +132,7 @@ def f
 def f
   %()
 end
+
+def foo(return:)
+  { return: }
+end

test/corpus/literal/send.rb

@@ -82,3 +82,5 @@ def foo
 foo&.!
 foo.~(b)
 a&.+(b)
+a = nil
+foo(bar: a)