GFM mode bold fontification incorrect for within-token underscores

[rriegs]

Expected Behavior

This issues is essentially the same as #448 but for bold fontification via double underscore (__). In GFM mode, like single asterisk and underscore, within-token double asterisk should fontify as bold while within-token double underscore should not fontify, i.e.

**strong**

st**ro**ng

**str**ong

str**ong**

**still strong**

**still str**ong

sti**ll strong**

sti**ll str**ong

__strong__

not__str__ong

__not__strong

not__strong__

__still strong__

no__t strong__

__not str__ong

no__t str__ong

should be (at least partially) fontified as their respective text suggests.

Actual Behavior

Like double asterisk, double underscore fontifies in all of the above cases.

Note: See also #524, which observes a bug when just one character is surrounded by single asterisks or underscores.

Software Versions

markdown-mode version 2.5-dev (installed via MELPA)
GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14) of 2020-03-26, modified by Debian
Ubuntu 20.04.1 LTS

[syohex]

I have merged #528. Please check latest version