function lowercaseKeys( obj ) {
	var out;
	var key;
	if ( typeof obj !== 'object' || obj === null ) {
		throw new TypeError( format( 'invalid argument. Must provide an object. Value: `%s`.', obj ) );
	}
	out = {};
	for ( key in obj ) {
		if ( hasOwnProp( obj, key ) ) {
			out[ key.toLowerCase() ] = obj[ key ];
		}
	}
	return out;
}
Converts each object key to lowercase.

@param {Object} obj - source object
@throws {TypeError} must provide an object
@returns {Object} new object

@example
var obj1 = {
    'A': 1,
    'B': 2
};

var obj2 = lowercaseKeys( obj1 );
// returns { 'a': 1, 'b': 2 }