Add test for mixed user info encoding

[dbu]

Description
In #67 we added tests about the url encoding of user info fields. However, in the discussion to define the expected behaviour in PSR-7, the concern has been raised that there can be unclear cases where some characters look like url encoded and others are not encoded

Example

Should the % in pa%65sw:ord be encoded? Most likely yes, but there is disagreement in the discussion on the PSR-7 what to do in such a case.