Underline in u and url tags in RichTextLabel don´t cover extra spacing while fill tag is enabled

[Bethleem]

Godot version: 3.1.1-stable_win32

OS/device including version: Windows 7/PC

Issue description: In RichTextLabel, when you have text between [fill] [/fill] tags, it sometimes adds extra spacing for the line to match the right edge. Well, this extra spacing is unkwon for [u] and [url] tags, that not put the underline under it and creates a ugly gap.

Steps to reproduce:
-Create a Rich text label.
-Add some text like "[Fill]Enough text to fill [u]the line and reach the [/u]right limit, so extra spacing may be added and NO UNDERLINED[/fill]"
-Enjoy the bug!

Minimal reproduction project:
Bbcode text in the example is: [fill]\t Se cree que [u]los primeros pobladores[/u][/fill]" and the output (see the not underlined gap between words "los" and "primeros"

[KoBeWi]

Still valid in 3.2.3

[bruvzg]

For the reference, it's fixed in master:

[akien-mga]

Seems still reproducible in 3.4 RC 1. Might be of interest to @timothyqiu 👀

[akien-mga]

Fixed by #54296.