Keep consistency on allow_blank and allow_nil qualifiers

After some discussion[1] was decided that these qualifiers do not accept
any parameter.

#722

NEWS.md

@@ -1,3 +1,15 @@
+# HEAD (unreleased)
+
+### Improvements
+
+* For consistency with other matchers, the `allow_nil` and `allow_blank`
+  qualifiers on the `validate_inclusion_of` matcher does not take any arguments;
+  using these qualifiers will always enable these options.
+
+  * *Pull request: [#747]*
+
+[#747]: https://github.com/thoughtbot/shoulda-matchers/pull/747
+
 # 3.1.1
 
 ### Bug fixes

lib/shoulda/matchers/active_model/validate_inclusion_of_matcher.rb

@@ -310,17 +310,17 @@ def in_range(range)
           self
         end
 
-        def allow_blank(allow_blank = true)
-          @options[:allow_blank] = allow_blank
+        def allow_blank
+          @options[:allow_blank] = true
           self
         end
 
         def expects_to_allow_blank?
           @options[:allow_blank]
         end
 
-        def allow_nil(allow_nil = true)
-          @options[:allow_nil] = allow_nil
+        def allow_nil
+          @options[:allow_nil] = true
           self
         end