fix: correctly run .matches when passing regex object

[tonysamperi]

Description

Forcing pattern to be string, to cope with lib validator which doesn't reset the RegExp instance
Fixes #1127
Workaround of a behaviour which should be fixed in validator.js

To-do list

• I have added tests for what I changed.

• This pull request is ready to merge.

[fedeci]

Merging and publishing this tomorrow morning! Sorry again for the delay!

[coveralls]

Coverage remained the same at 100.0% when pulling 8c4059a on tonysamperi:bugfix/regex-instance into 82e4d84 on express-validator:master.