Add some tests for uris

[bghgary]

See KhronosGroup/glTF#1449

Might consider adding other uri encoding edge cases as well for completeness.

[emackey]

My suggestion:

• Test a URI with an encoded %20 space
• Test a URI with an un-encoded ' ' space
• Test various other encoded characters that are allowable as filename characters on most filesystems (maybe $ might be a good test?)

No need to test encoded characters that are illegal in popular filesystems.

[kcoley]

Some more for images:

• Test a URI with an embedded base64 texture
• Test a URI with an embedded base64 buffer