bugfix: allow string array to be passed to auth guard

[michaeljota]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #67

What is the new behavior?

AuthGuard function and defaultStrategy both allow a string array to be set.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

This PR does not break any existing code, but would allow several strategies to be set.

Other information

[michaeljota]

The force push was to update the commit to follow the commit style guide.

[kamilmysliwiec]

Thank you!