Add tests for `scan_uri_escapes`

[Paalon]

This function is not tested because, in spite of it uses Julia v0.3 function char, which is not available in Julia v0.4, we cannot observe related errors in our test sets.

https://github.com/JuliaData/YAML.jl/blob/45f5cd2f43fe4ac424cc4f9615c8bdff560f407c/src/scanner.jl#L1580

[kescobo]

does it need a convert(Char...) or something?

[kescobo]

I see you addressed in #164