Given markdown;
# a

a
=

## b

b
-

Execute (SetexToAtx):
  :SetexToAtx

Expect (convert setex-style headings to atx):
  # a

  # a

  ## b

  ## b

Given markdown;
a
=

b
=

c
-

d
-

Execute (SetexToAtx with range):
  :1,8SetexToAtx

Expect (only convert setex headings in original range):
  # a

  # b

  ## c

  d
  -