[PostgresAdmin] Fix backup_type being ignored

With the previous implementation, the code would always attempt to find
the magic number of the file for a `pg_dump` before checking if the
`backup_type` value was instead a `:basebackup`.  When streaming a file,
this would cause it to fail farther down the line since it would have
strip some of the content off from the beginning of the file,
invalidating it as a proper backup.

This change fixes by forcing the check for the backup type first, and
then by inspecting the file contents.

lib/gems/pending/util/postgres_admin.rb

@@ -99,10 +99,12 @@ def self.backup(opts)
   def self.restore(opts)
     file        = opts[:local_file]
     backup_type = opts.delete(:backup_type)
-    if backup_type == :pgdump || pg_dump_file?(file)
-      restore_pg_dump(opts)
-    elsif backup_type == :basebackup || base_backup_file?(file)
-      restore_pg_basebackup(file)
+
+    case
+    when backup_type == :pgdump     then restore_pg_dump(opts)
+    when backup_type == :basebackup then restore_pg_basebackup(file)
+    when pg_dump_file?(file)        then restore_pg_dump(opts)
+    when base_backup_file?(file)    then restore_pg_basebackup(file)
     else
       raise "#{file} is not a database backup"
     end