Added Faker::Code.isbn

spec/code_spec.cr

@@ -0,0 +1,17 @@
+require "./spec_helper"
+
+describe Faker::Code do
+  it "base 10 isbn" do
+    Faker::Code.isbn.match(/^\d{9}-[\d|X]$/).should_not eq nil
+  end
+
+  it "base 13 isbn" do
+    Faker::Code.isbn(13).match(/^\d{12}-\d$/).should_not eq nil
+  end
+
+  it "should return deterministic results when seeded" do
+    Faker.seed 123456
+    Faker::Code.isbn.should eq "394314441-0"
+    Faker::Code.isbn(13).should eq "205982563728-9"
+  end
+end