db.runCommand() should return error if second argument is invalid

Team,

If we try to create index using misplaced background configuration, MongoDB should be able to validate and report back the error. For example, if I run following command, server doesn't tells me that background is placed wrong and instead runs the indexing in foreground:

db.runCommand({

	createIndexes: "exampleIndex",

	indexes: [{

		key: {

			a: 1,

			b: 1,

			"c": 1

		},

		name: "a_1_b_1_c_1"

	},

	{

		key: {

			a: 1,

			b: 1,

			c: 1,

			"d": 1,

			e: 1

		},

		name: "a_1_b_1_c_1_d_1_e_1"

	}]

},{ background: true })

Note that the background: true is not present per index level but is defined at global level.