Don't clobber `in` and `name` fields in body/form parameters

Fixes #2.  I'll try to follow this up with a corresponding test script.
The spec's [Parameter Object fixed fields][0] section says that `name` and `in` are required fields for the parameter, however in the preceding section regaring [parameter types][1], `name` is ignored for the `body` type, being used instead just as documentation marker.  So yeah, checking the body of the request itself seems to be the way to go here.

[0]: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#fixed-fields-7
[1]: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#parameter-object-