Allow locations kwarg for jwt_required() to be a string

[Tloch95]

If only specifying a single location to check for JWT in the jwt_required() decorator, it should be acceptable to pass a string instead of a list for the "locations" kwarg.

Example:
Instead of @jwt_required(locations=['cookies'])...
@jwt_required(locations='cookies') should also be acceptable

[vimalloc-mavenlink]

Awesome, thanks for the report! We will get that fixed up and a new version released here soon! 👍