URLdecode attributes Parsoid feeds us

Title class tries to build the object based on the DBkey we get from Parsoid's
data. It looks like not everything in there is urldecoded, which makes
Title::newFromDBkey fail (return null). We didn't check for the null case, which
led to a fatal error.

The exact value we were failing on is:
File:Wolfgang_Amadeus_Mozart_-_Sinfonia_Concertante_für_Violine%2C_Viola_und_Orchester_-_3._Presto.ogg

Bug: 65129
Change-Id: Icfc989cc773f4dadeb8f24f1b8e2447af8b400d3