Only use and/or keywords for control flow (#12)

Using `and` and `or` for anything other than control flow has too often lead to
subtle bugs.

lib/pagy.rb

@@ -22,7 +22,7 @@ def self.root; Pathname.new(__FILE__).dirname end
 
   # merge and validate the options, do some simple aritmetic and set the instance variables
   def initialize(vars)
-    @vars = VARS.merge(vars.delete_if{|k,v| v.nil? or v == '' })          # default vars + cleaned instance vars
+    @vars = VARS.merge(vars.delete_if{|k,v| v.nil? || v == '' })          # default vars + cleaned instance vars
     { count:0, items:1, outset:0, page:1 }.each do |k,min|                # validate core variables
       (@vars[k] && instance_variable_set(:"@#{k}", @vars.delete(k).to_i) >= min) \
          or raise(ArgumentError, "expected :#{k} >= #{min}; got #{instance_variable_get(:"@#{k}").inspect}")