fix positional parameter warnings

Author: mamantoha

src/clear/expression/expression.cr

@@ -83,7 +83,7 @@ class Clear::Expression
       @value
     end
 
-    def to_json(b = nil)
+    def to_json(x = nil)
       @value
     end
   end

src/clear/extensions/enum/enum.cr

@@ -20,8 +20,8 @@ module Clear
       @value.to_sql
     end
 
-    def to_json(json)
-      json.string(@value)
+    def to_json(x)
+      x.string(@value)
     end
 
     def ==(x)