Fix for getRelativeURL & URI character coercer

[eblondel]

Dear @duncantl, here's a proposal of fix for getRelativeURL & the setAs character method for URI objects. I've raised issues while doing attempts to parse local XSD schemas (schemas from https://github.com/eblondel/geometa/tree/master/inst/extdata/schemas) with the XMLSchema package. The issues were:

• with getRelativeURL: handling with relative paths with ../ occurences was not working, leading to bad handling of XSD imports/includes.
• with setAs("URI", "character" coercer where with local file paths I was getting port = -1 leading to a bad handling of imports/includes paths. Thanks in advance if you can review this, and hopefully merge it if code is fine.

[eblondel]

@omegahat @duncantl could you please have a look to this minor pull request, this would help a lot having this fix in XMLSchema package. Thanks a lot in advance for your support