Modify Headers plugin fails if path is named with preceding slash

[ghost]

internal issue: https://github.com/TykTechnologies/tyk-analytics/issues/663

Do you want to request a feature or report a bug?
bug

What is the current behavior?
In the Endpoint Designer, the Modify Header plugin won't work for paths named with a preceding slash. E.g. "/v2/pets/5" won't work, but "v2/pets/5" will.

What is the expected behavior?
It should be agnostic of the preceding slash, like the other plugins.

[joshblakeley]

#1515 fixed this