AttributeError exception in _example_from_array_spec() when items specified as array of types

[mtherieau]

in cases when an array's items are specified in so-called "tuple mode", _example_from_array_spec() throws at AttributeError exception like so:
https://travis-ci.org/Trax-air/swagger-parser/jobs/214411452#L878

reference: so-called "tuple validation" https://spacetelescope.github.io/understanding-json-schema/reference/array.html

Test case added in: #39
an example of this issue is in file: tests/array_items_list.yaml

fix is part of same PR here:
1569e29