KeyError exception in _example_from_complex_def() when inner object has no schema field

[mtherieau]

in cases where there is no schema field (e.g. an inner object) then _example_from_complex_def() throws a KeyError exception, like so:

https://travis-ci.org/Trax-air/swagger-parser/jobs/214411452#L693

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

c80700d#diff-3b7092222dd5ce0fb27f2f2841aa17d1

fix is part of same PR, here:
a99373b