fix: serialize attributes through quoteAttribute in removeWhitespace

[greymoth-jp]

removeWhitespace() rebuilds rawAttrs to drop the whitespace between attributes, but it serializes each value with JSON.stringify(val) instead of going through quoteAttribute, the helper the other write paths use. That leaves it with the two bugs the helper already handles, so removeWhitespace() corrupts attribute values:

• Consecutive backslashes get doubled. <a data-x="C:\Users\me"> becomes <a data-x="C:\\Users\\me"> and no longer round-trips. This is the same problem fixed for the setter paths in quoteAttribute corrupts attribute values containing '\\' / '\t' / '\n' — round-trip parse(el.toString()) lossy #306 and fix: preserve consecutive backslashes in attribute values #309 (JSON.stringify doubling backslashes), but removeWhitespace kept the old serialization.
• Valueless attributes gain a null value. <input disabled required> becomes <input disabled=null required=null>, because JSON.stringify(null) is the string "null". setAttribute/removeAttribute already guard this with if (val === 'null' || val === '""') return name.

The fix routes the value through quoteAttribute and applies the same valueless-attribute guard the three other serialization sites (setAttribute, removeAttribute, setAttributes) already use, so all four paths agree.

I added two regression tests under #removeWhitespace(). Without the change they fail with the corrupted output above; with it the suite passes.