Menutree bootstrap dropdown fix

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1883
| Refs tickets  | -
| License       | MIT
| Doc PR        | -
This should be `!==`, because `strpos('dropdown', 'dropdown') != false` is false whereas  `strpos('dropdown', 'dropdown') !== false` is true.