TextRun#text= on a run without w:t leaves #text stale #190

When a run has no w:t, TextRun#text= creates one and writes the content into the XML. But @text_nodes isn't refreshed, so #text (and #to_s, #to_html) still returns "".

Repro

w = 'http://schemas.openxmlformats.org/wordprocessingml/2006/main'
run = Docx::Elements::Containers::TextRun.new(Nokogiri::XML(%(<w:r xmlns:w="#{w}"/>)).root)
run.text = 'new'
run.text       # => ""     (expected "new")
run.node.text  # => "new"

Runs created through TextRun.create_with / create_within start out like this, so anything that sets their text and then reads the same object back sees the stale value.

Expected: #text returns "new". For example, text= could append the new Elements::Text to @text_nodes before reset_text.

Found while splitting the specs into per-class unit specs (#188).