Reader should validate

[jcoyne]

https://github.com/projecthydra/ldp/blob/master/lib/ldp/resource/rdf_source.rb#L57

We're seeing errors like this:

ERROR [line: 42] undefined prefix "ns002"
ERROR [line: 43] undefined prefix "ns002"

Which are resulting in data loss (mostly silently). If we validate, this would instead raise an error, which would be more noticeable.

Validate option is documented here: https://github.com/ruby-rdf/rdf-turtle/blob/316ec78d048362d26a565845a1a6f9e2bf899024/lib/rdf/turtle/reader.rb#L246