The `upper` template filter should set `is_safe=True`

[Lily Foote]

I noticed this inconsistency between upper and lower where upper has is_safe=False but lower has is_safe=True. I think lower is correct and upper is incorrect.

This has been the case ever since is_safe was introduced in ​https://github.com/django/django/commit/356662cf74c99fac90afb0f5e6aac8d2d573e62a.

from django.template import engines
from django.utils.safestring import mark_safe


def test_is_safe():
    template = """
{{ html|upper }}
{{ html|lower }}
"""
    template = engines["django"].from_string(template)
    html = mark_safe("<p>Hello World!</p>")
    assert template.render({"html": html}) == """
<P>HELLO WORLD!</P>
<p>hello world!</p>
"""

E       AssertionError: assert '\n&lt;P&gt;H... world!</p>\n' == '\n<P>HELLO W... world!</P>\n'
E         
E           
E         - <P>HELLO WORLD!</P>
E         + &lt;P&gt;HELLO WORLD!&lt;/P&gt;
E           <p>hello world!</p>

[Chaitanya Rahalkar]

See ​https://github.com/django/django/pull/18988