Allow passing nil in as a variable value

Closes #70

This requires moving things around so that nil values are only set to
the nodes' context hash if that value was explicitly set on the
calculator. Previously every referenced variable name would be added as
a key to the context hash even it was unbound.

lib/dentaku/ast/identifier.rb

@@ -10,12 +10,13 @@ def initialize(token)
       end
 
       def value(context={})
-        v = context[identifier]
+        v = context.fetch(identifier) do
+          raise UnboundVariableError.new([identifier])
+        end
+
         case v
         when Node
           v.value(context)
-        when NilClass
-          raise UnboundVariableError.new([identifier])
         else
           v
         end

lib/dentaku/bulk_expression_solver.rb

@@ -43,8 +43,18 @@ def raise_exception_handler
     def load_results(&block)
       variables_in_resolve_order.each_with_object({}) do |var_name, r|
         begin
-          r[var_name] = calculator.memory[var_name] ||
-                        evaluate!(expressions[var_name], expressions.merge(r))
+          value_from_memory = calculator.memory[var_name]
+
+          if value_from_memory.nil? &&
+              expressions[var_name].nil? &&
+              !calculator.memory.has_key?(var_name)
+            next
+          end
+
+          value = value_from_memory ||
+            evaluate!(expressions[var_name], expressions.merge(r))
+
+          r[var_name] = value
         rescue Dentaku::UnboundVariableError, ZeroDivisionError => ex
           r[var_name] = block.call(ex)
         end

spec/calculator_spec.rb

@@ -264,6 +264,22 @@
     it 'can be used in IF statements' do
       expect(calculator.evaluate('IF(null, 1, 2)')).to eq(2)
     end
+
+    it 'can be used in IF statements when passed in' do
+      expect(calculator.evaluate('IF(foo, 1, 2)', foo: nil)).to eq(2)
+    end
+
+    it 'nil values are carried across middle terms' do
+      results = calculator.solve!(
+        choice: 'IF(bar, 1, 2)',
+        bar: 'foo',
+        foo: nil)
+      expect(results).to eq(
+        choice: 2,
+        bar: nil,
+        foo: nil
+      )
+    end
   end
 
   describe 'case statements' do