fix(rsx): formatted attribute merging

[sehnryr]

When attributes are merged in Element::merge_attributes, out's source is not updated (I don't know if it should) and even if the segments are trivial, since source is empty we cannot consider it as a "simple expression" by is_simple_expr.

This would result in an issue when merging the following classes with me output being empty:

div {
    class: "foo",
    class: "{bar}",
}

I also modified the merges_attributes test splitting it into different cases:

• merge_trivial_attributes: merge between two literals
• merge_formatted_attributes: merge between a literal and a formatted
• merge_conditional_attributes: merge between a literal and two conditionals
• merge_all_attributes: merge between all of the above