Exception thrown when security requirement key doesn't match a security scheme definition

[PerthCharern]

Exception thrown when security requirement key doesn't match a security scheme definition.

We should parse the document normally, but yield a validation error instead of throwing.

[senthilkumarmohan]

Hi, I have checked in fix for this issue #339

[PerthCharern]

@senthilkumarmohan Thanks for the PR. I'll take a look.