Autocomplete sometimes makes it impossible to type exact domains #1292

To reproduce:

• start Servo
• type "reddit.com"
• in the edit box, "reddit.com over 18" appears, with the "over 18" part highlighted
• erase "over 18" by pressing backspace
• press Enter
• a search page opens, with "reddit.com over 18" as a query

I was asked to re-file this here, over from servo/servo.