Raise an exception when getting out of bounds includes

Fixes #5 
`json_include` specifying an index where nothing exists was the underlying problem. I have chosen to throw an error in that case, because nobody ought to be *trying* to get `nil` from that method. Since we are testing, a nice error message would probably be helpful.

I also added an error class so that expectations in this repo testing that an error is thrown when it should be are less likely to give a false positive.

Author: happycollision

lib/jsonapi_spec_helpers.rb

@@ -3,6 +3,7 @@
 require 'jsonapi_spec_helpers/helpers'
 require 'jsonapi_spec_helpers/payload'
 require 'jsonapi_spec_helpers/payload_sanitizer'
+require 'jsonapi_spec_helpers/error'
 
 module JsonapiSpecHelpers
   def self.included(klass)

lib/jsonapi_spec_helpers/error.rb

@@ -0,0 +1,2 @@
+class JsonapiSpecHelpersError < StandardError
+end

lib/jsonapi_spec_helpers/helpers.rb

@@ -39,7 +39,12 @@ def json_includes(type, *indices)
       indices  = (0...included.length).to_a if indices.empty?
       includes = []
       indices.each do |index|
-        includes << json_item(from: included.at(index))
+        single_included = included.at(index)
+        if single_included.nil?
+          raise JsonapiSpecHelpersError,
+            included_out_of_bounds_error(type, index, included)
+        end
+        includes << json_item(from: single_included)
       end
       includes
     end
@@ -96,4 +101,12 @@ def jsonapi_payload(input)
       PayloadSanitizer.new(input).sanitize
     end
   end
+
+private
+
+  def included_out_of_bounds_error(type, index, array)
+    "You attempted to get an item at index #{index} of the type '#{type}' " \
+    "from the included property of your JSON payload. But it contained "    \
+    "#{array.length} '#{type}'"
+  end
 end