models: don't attempt to format invalid phone numbers

This is a defensive-programming measure, because formatting an
invalid phone number may truncate some leading numbers.

app/models/champs/phone_champ.rb

@@ -50,6 +50,13 @@ class Champs::PhoneChamp < Champs::TextChamp
 
   def to_s
     return '' if value.blank?
+
+    if Phonelib.valid_for_countries?(value, DEFAULT_COUNTRY_CODES)
       Phonelib.parse_for_countries(value, DEFAULT_COUNTRY_CODES).full_national
+    else
+      # When he phone number is possible for the default countries, but not strictly valid,
+      # `full_national` could mess up the formatting. In this case just return the original.
+      value
+    end
   end
 end

spec/models/champs/phone_champ_spec.rb

@@ -52,6 +52,12 @@ describe Champs::PhoneChamp do
         expect(champ_with_value("45187272").to_s).to eq("45187272")
       end
     end
+
+    context 'for possible (but not valid) phone numbers' do
+      it 'returns the original' do
+        expect(champ_with_value("1234").to_s).to eq("1234")
+      end
+    end
   end
 
   def champ_with_value(number)