Fixed a test bug in DefaultBinderFixture

The test didn't specify any body deserializers, so it didn't really test
what it said it tested.
Adds BindingConfig.BodyOnly

The BodyOnly setting instructs DefaultBinder to only bind to the body of a
request, and NOT continue with form fields, request parameters etc. This
means that you can have a request parameter with the same name as a model
property without the model property being overwritten by the parameter.
Updated the documentation for BindingConfig.BodyOnly.
Merge pull request #1031 from provegard/modelbind-bodyonly

New binding configuration option: BodyOnly