Rename FileBody to HttpBody

Now that our custom body type does things more than files, it makes
sense to rename it.  This commit renames the header itself, then all
instances of the class.

Tested: Basic GET requests succeed.
Change-Id: If4361ac8992fc7c268f48a336707f96e68d3576c
Signed-off-by: Ed Tanous <ed@tanous.net>
diff --git a/meson.build b/meson.build
index d644b79..487ec1f 100644
--- a/meson.build
+++ b/meson.build
@@ -431,7 +431,7 @@
   'test/http/verb_test.cpp',
   'test/http/server_sent_event_test.cpp',
   'test/http/mutual_tls_meta.cpp',
-  'test/http/http_file_body_test.cpp',
+  'test/http/http_body_test.cpp',
   'test/include/dbus_utility_test.cpp',
   'test/include/google/google_service_root_test.cpp',
   'test/include/json_html_serializer.cpp',