Import changes from ruby-build v20150112

plugins/python-build/test/test_helper.bash

@@ -115,6 +115,10 @@ assert_output() {
 
 assert_output_contains() {
   local expected="$1"
+  if [ -z "$expected" ]; then
+    echo "assert_output_contains needs an argument" >&2
+    return 1
+  fi
   echo "$output" | $(type -p ggrep grep | head -1) -F "$expected" >/dev/null || {
     { echo "expected output to contain $expected"
       echo "actual: $output"