restore setting the default branch for build requests (#1082)

Author: svenfuchs

lib/travis/api/v3/queries/request.rb

@@ -35,7 +35,7 @@ def schedule(repository, user)
         },
         id: request.id,
         message: message,
-        branch: branch,
+        branch: branch || repository.default_branch.name,
         sha: sha,
         merge_mode: merge_mode,
         config: config

spec/v3/services/requests/create_spec.rb

@@ -133,7 +133,7 @@
       },
       id: request.id,
       message: nil,
-      branch: nil,
+      branch: 'master',
       sha: nil,
       merge_mode: nil,
       config: nil