Fix percent-encoded fragment modification in Poco::URI

[dahek]

Before this commit using Poco::URI class to parse specific URIs that had
percent-encoded fragment identifier resulted in the loss of information
concerning the way the fragment identifier was encoded. There could be the cases
when the result of Poco::URI object serialization to string did not match the
original URI string Poco::URI object was created from.

In this commit we change the internal logic of fragment processing in Poco::URI,
so that the fragment is stored inside the class in raw form (the same way as
query string). The methods getFragment and setFragment work the old way (with
percent-decoded fragment values), new methods getRawFragment and setRawFragment
are added to get access to the original fragment representation.

[aleks-f]

@obiltschnig this is a breaking change - getFragment() would now returns const std::string&, so that would now be a reference to a temporary object for code that was retrieving it as reference