Nested derivatives fail

**Describe the bug**
mathJs evaluates this derivative correctly:
`derivative('x * e^x', "x")` // returns "e ^ x * (x + 1)"

however, if the expression to be differentiated is involves another derivative mathJs returns an incorrect answer:
`derivative('x * derivative("e^x", "x")', "x")` // returns "e^x"

**To Reproduce**
enter expression in mathJs demo window:
`derivative('x * derivative("e^x", "x")', "x")`