add a test case for a query without a value (java's URI supports that)

Parsing URLs like ```http://theon.github.com/uris-in-scala.html?ham``` throws an exception (I don't have a fix, sorry)
```
org.parboiled2.ParseError was thrown.
org.parboiled2.ParseError
	at org.parboiled2.Parser.buildParseError$1(Parser.scala:170)
	at org.parboiled2.Parser.__run(Parser.scala:182)
	at com.netaporter.uri.parsing.UriParser$.parse(UriParser.scala:167)
	at com.netaporter.uri.Uri$.parse(Uri.scala:271)
```