handle pcnt-encoded names in templates

[marc-portier]

Between 0.5 and 0.6 there was some change in how this should be handled.

The inserted snippet is:

A varname MAY contain one or more pct-encoded triplets. These
triplets are considered an essential part of the variable name and
are not decoded during processing. A varname containing pct-encoded
characters is not the same variable as a varname with those same
characters decoded. Applications that provide URI Templates are
expected to be consistent in their use of pct-encoding within
variable names.

The task is to understand this, translate it into (updated) tests and then change the implementation accordingly.