Regression: <spacer> no longer including extra attributes

[marcandre]

The <spacer> tag no longer includes extra attributes and they are entirely dropped:

Example:

<spacer class="some classes" foo="bar" size="42"/>
# 1.3.8 converts to:
<table foo="bar" class="classes some spacer">[...]
# 1.4.2 converts to:
<table class="classes some spacer">[...]

Note that other tags do include extra attributes.

(Noticed when running the inky-rb test suite)