add tests for urlencoded characters

[untitaker]

Assertions:

• that special characters are not urlencoded in folder listing
• that they are correctly handled for GET/PUT to a file
• maybe that nullbytes in node names are rejected?
• that arbitrary bytes work in files

cc @fkooman @skddc

[untitaker]

Using something like hypothesis would be nice for those purposes, or some fuzzing library.