(feat) - Add merge_facts option

This commit adds an option to the add_custom_fact method, which allows
the user to specify whether they wish to deep_merge facts.

This allows the user to determine whether they wish to completely
overwrite a fact, or simply merge values into the existing fact in a
given fact hash.

README.md

@@ -443,6 +443,28 @@ To do this, pass a lambda as the value for the custom fact. The lambda is passed
 add_custom_fact :root_home, lambda { |os,facts| "/tmp/#{facts['hostname']}" }
 ```
 
+#### Merge into existing facts
+
+You can also supply an optional input `:merge_facts` to the `add_custom_fact` method.
+
+This allows you to merge facts values into a fact, if the fact is already present in the facts hash as oppose to overwriting the fact value.
+
+```ruby
+add_custom_fact :identity, { 'user' => 'test_user' }, :merge_facts => true
+```
+
+Will result in a hash of the identity fact like the below:
+
+```ruby
+{
+  "gid"=>0,
+  "group"=>"root",
+  "privileged"=>true,
+  "uid"=>0,
+  "user"=>"test_user"
+}
+```
+
 ### Supplying Custom External Facts through FacterDB
 Rspec-puppet-facts uses a gem called facterdb that contains many fact sets of various combinations that are pre generated.  Rspec-puppet-facts queries
 facterdb to pull out a specific fact set to use when testing.

lib/rspec-puppet-facts.rb

@@ -229,7 +229,13 @@ def self.with_custom_facts(os, facts)
       next if fact[:options][:confine] && !fact[:options][:confine].include?(os)
       next if fact[:options][:exclude] && fact[:options][:exclude].include?(os)
 
-      facts[name] = fact[:value].respond_to?(:call) ? fact[:value].call(os, facts) : fact[:value]
+      value = fact[:value].respond_to?(:call) ? fact[:value].call(os, facts) : fact[:value]
+      # if merge_facts passed, merge supplied facts into facts hash
+      if fact[:options][:merge_facts]
+        facts.deep_merge!({name => value})
+      else
+        facts[name] = value
+      end
     end
 
     facts