(PUP-7130) Fix handling of DateTime.httpdate(x).to_time

In Ruby versions prior <2.4.0, the statement
```
x = DateTime.httpdate('Sat, 03 Feb 2001 04:05:06 GMT').to_time.to_s
```
produces a local time like "2001-02-03 05:05:06 +0100". In 2.4.0 the
timezone is retained. This commit ensures that the time used for the
http_metadata checksum is UTC and that the test expects UTC. That works,
regardless of Ruby version.

lib/puppet/file_serving/http_metadata.rb

@@ -23,7 +23,7 @@ def initialize(http_response, path = '/dev/null')
 
     if last_modified = http_response['last-modified']
       mtime = DateTime.httpdate(last_modified).to_time
-      @checksums[:mtime] = "{mtime}#{mtime}"
+      @checksums[:mtime] = "{mtime}#{mtime.utc}"
     end
 
     @ftype = 'file'

spec/unit/file_serving/http_metadata_spec.rb

@@ -61,7 +61,7 @@
         metadata = described_class.new(http_response)
         metadata.collect
         expect( metadata.checksum_type ).to eq :mtime
-        expect( metadata.checksum ).to eq "{mtime}#{time.to_time}"
+        expect( metadata.checksum ).to eq "{mtime}#{time.to_time.utc}"
       end
     end