Upgrade http.rb gem to 5.0.4.

Fixes a bug where the Foundation source strategy failed because http.rb
automatically sent a `Content-Length: 0` header with all GET requests,
which caused Foundation to return a 400 Bad Request error. This behavior
was fixed in http.rb 5.x.

http.rb 5.x has a breaking change where it now includes the request object
inside the response object, which we have to handle in a few places.

Gemfile.lock

@@ -266,14 +266,12 @@ GEM
     heapy (0.2.0)
       thor
     hsluv (1.0.1)
-    http (4.4.1)
-      addressable (~> 2.3)
+    http (5.0.4)
+      addressable (~> 2.8)
       http-cookie (~> 1.0)
       http-form_data (~> 2.2)
-      http-parser (~> 1.2.0)
+      llhttp-ffi (~> 0.4.0)
     http-form_data (2.3.0)
-    http-parser (1.2.3)
-      ffi-compiler (>= 1.0, < 2.0)
     httpclient (2.8.3)
     i18n (1.9.1)
       concurrent-ruby (~> 1.0)
@@ -290,6 +288,9 @@ GEM
     listen (3.7.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
+    llhttp-ffi (0.4.0)
+      ffi-compiler (~> 1.0)
+      rake (~> 13.0)
     loofah (2.14.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
@@ -568,7 +569,7 @@ DEPENDENCIES
   google-cloud-bigquery
   google-cloud-storage
   hsluv
-  http (~> 4.4.1)
+  http
   http-cookie!
   ipaddress_2
   listen