The iteration of reading the input string of the function was
problematic. When finding a pre-encoded character, for instance
%28 (which is an open bracket - '('), all three characters were
written in the destination string, but the reading of the input
string continued with the character after '%', which was '2'. The
correct behavior is for the reading to continue with the character
after '8'.

The writing in the destination also had a bug. Instead of writing
the 3 characters ('%', '2' and '8') one after another, the first
two were written correctly, namely at position j and j+1 in the
destination string, but the third character was written at j+3
instead of at j+2.

This problem had not appeared before in the tests in part_test,
because there was no test case that has both a pre-encoded
character and a character to be encoded. When there are no
characters to be encoded in the input string, the string is
returned as it was given in the input. Such a test case would have
resulted in an index out of range error.

…scaping

Fix escaping in part name normalization

Fix TestPart_validate