Fix to_string() method and change replacement constructor

[magdacisowska]

I noticed that

 auto original = uri{some_string};
 auto copy1 = original; // copy assignment
 auto copy2 = uri{original, {}}; // replacement ctor

results in copy1 != copy2, and therefore copy1.to_string() != copy2.to_string(). I believe this is not the desired behaviour.

Also I didn't understand this condition in to_string() method: if (m_content.length() > m_path.length()) so I made it m_category dependent.