Fix build parameters POST body

- Restore build_parameters key in build_branch POST body

lib/circleci/project.rb

@@ -277,11 +277,13 @@ def build
     #
     # Build the latest push for this branch of a specific project
     #
-    # @param branch   [String] - Name of branch
-    # @param params   [Hash] - Optional params to send for building
-    # @param body     [Hash] - Optional post body with build parameters
-    # @return         [CircleCi::Response] - Response object
-    def build_branch(branch, params = {}, body = {})
+    # @param branch             [String] - Name of branch
+    # @param params             [Hash] - Optional params to send for building
+    # @param build_parameters   [Hash] - Optional post body with build parameters
+    # @return                   [CircleCi::Response] - Response object
+    def build_branch(branch, params = {}, build_parameters = {})
+      body = {}
+      body['build_parameters'] = build_parameters unless build_parameters.empty?
       CircleCi.request(@conf, "/project/#{username}/#{project}/tree/#{branch}", params).post(body)
     end