Fix #68 and support use of opaque base uris

e.g. this is neccessary to support working with csvw inside a zip file

jar:file:///some/path.zip!/inside/zip/metadata.json

Though that usecase also requires some additional extensions to
csv2rdf protocols.

src/csv2rdf/util.clj

@@ -17,7 +17,9 @@
 (defn set-fragment
  "Sets the fragment on a URI to the given value."
  [^URI uri fragment]
- (URI. (.getScheme uri) (.getUserInfo uri) (.getHost uri) (.getPort uri) (.getPath uri) (.getQuery uri) fragment))
+ (if (.isOpaque uri)
+ (URI. (.getScheme uri) (.getSchemeSpecificPart uri) fragment)
+ (URI. (.getScheme uri) (.getUserInfo uri) (.getHost uri) (.getPort uri) (.getPath uri) (.getQuery uri) fragment)))
 
 (defn remove-fragment
  "Sets the fragment on a URI to nil."